diff --git a/apisix/core/string.lua b/apisix/core/string.lua
index 952152a8..51d1577a 100644
--- a/apisix/core/string.lua
+++ b/apisix/core/string.lua
@@ -36,6 +36,7 @@ setmetatable(_M, {__index = string})
 
 
 -- find a needle from a haystack in the plain text way
+-- note: Make sure that the haystack is 'string' type, otherwise an exception will be thrown.
 function _M.find(haystack, needle, from)
     return str_find(haystack, needle, from or 1, true)
 end
diff --git a/apisix/discovery/eureka.lua b/apisix/discovery/eureka.lua
index 179d763f..82ee89a1 100644
--- a/apisix/discovery/eureka.lua
+++ b/apisix/discovery/eureka.lua
@@ -28,7 +28,7 @@ local ngx                = ngx
 local ngx_timer_at       = ngx.timer.at
 local ngx_timer_every    = ngx.timer.every
 local string_sub         = string.sub
-local string_find        = string.find
+local str_find           = core.string.find
 local log                = core.log
 
 local default_weight
@@ -76,9 +76,9 @@ local function service_info()
     local basic_auth
     -- TODO Add health check to get healthy nodes.
     local url = host[math_random(#host)]
-    local auth_idx = string_find(url, "@", 1, true)
+    local auth_idx = str_find(url, "@")
     if auth_idx then
-        local protocol_idx = string_find(url, "://", 1, true)
+        local protocol_idx = str_find(url, "://")
         local protocol = string_sub(url, 1, protocol_idx + 2)
         local user_and_password = string_sub(url, protocol_idx + 3, auth_idx - 1)
         local other = string_sub(url, auth_idx + 1)
diff --git a/apisix/plugins/batch-requests.lua b/apisix/plugins/batch-requests.lua
index dfe0dfde..fc4dc64f 100644
--- a/apisix/plugins/batch-requests.lua
+++ b/apisix/plugins/batch-requests.lua
@@ -20,7 +20,7 @@ local plugin    = require("apisix.plugin")
 local ngx       = ngx
 local ipairs    = ipairs
 local pairs     = pairs
-local str_find  = string.find
+local str_find  = core.string.find
 local str_lower = string.lower
 
 
@@ -163,7 +163,7 @@ local function set_common_header(data)
 
         if outer_headers then
             for k, v in pairs(outer_headers) do
-                local is_content_header = str_find(k, "content-", 1, true) == 1
+                local is_content_header = str_find(k, "content-") == 1
                 -- skip header start with "content-"
                 if not req.headers[k] and not is_content_header then
                     req.headers[k] = v
diff --git a/apisix/plugins/cors.lua b/apisix/plugins/cors.lua
index 3251729f..e4cdbd37 100644
--- a/apisix/plugins/cors.lua
+++ b/apisix/plugins/cors.lua
@@ -17,7 +17,7 @@
 local core        = require("apisix.core")
 local ngx         = ngx
 local plugin_name = "cors"
-local str_find    = string.find
+local str_find    = core.string.find
 local re_gmatch   = ngx.re.gmatch
 
 
@@ -86,7 +86,7 @@ local _M = {
 
 
 local function create_mutiple_origin_cache(conf)
-    if not str_find(conf.allow_origins, ",", 1, true) then
+    if not str_find(conf.allow_origins, ",") then
         return nil
     end
     local origin_cache = {}
diff --git a/apisix/plugins/ip-restriction.lua b/apisix/plugins/ip-restriction.lua
index 2dead12c..b1860c91 100644
--- a/apisix/plugins/ip-restriction.lua
+++ b/apisix/plugins/ip-restriction.lua
@@ -18,7 +18,7 @@ local ipairs    = ipairs
 local core      = require("apisix.core")
 local ipmatcher = require("resty.ipmatcher")
 local str_sub   = string.sub
-local str_find  = string.find
+local str_find  = core.string.find
 local tonumber  = tonumber
 local lrucache  = core.lrucache.new({
     ttl = 300, count = 512
@@ -69,7 +69,7 @@ local _M = {
 
 local function valid_ip(ip)
     local mask = 0
-    local sep_pos = str_find(ip, "/", 1, true)
+    local sep_pos = str_find(ip, "/")
     if sep_pos then
         mask = str_sub(ip, sep_pos + 1)
         mask = tonumber(mask)
diff --git a/apisix/plugins/log-rotate.lua b/apisix/plugins/log-rotate.lua
index de2266cc..217c2f41 100644
--- a/apisix/plugins/log-rotate.lua
+++ b/apisix/plugins/log-rotate.lua
@@ -25,6 +25,7 @@ local io = io
 local os = os
 local table = table
 local string = string
+local str_find = core.string.find
 local local_conf
 
 
@@ -57,7 +58,7 @@ end
 
 local function get_last_index(str, key)
     local rev = string.reverse(str)
-    local _, idx = string.find(rev, key, 1, true)
+    local _, idx = str_find(rev, key)
     local n
     if idx then
         n = string.len(rev) - idx + 1
diff --git a/apisix/plugins/proxy-rewrite.lua b/apisix/plugins/proxy-rewrite.lua
index d24d0b42..1a5e7017 100644
--- a/apisix/plugins/proxy-rewrite.lua
+++ b/apisix/plugins/proxy-rewrite.lua
@@ -22,7 +22,7 @@ local ngx         = ngx
 local type        = type
 local re_sub      = ngx.re.sub
 local sub_str     = string.sub
-local find_str    = string.find
+local str_find    = core.string.find
 
 local schema = {
     type = "object",
@@ -158,7 +158,7 @@ function _M.rewrite(conf, ctx)
         end
     end
 
-    local index = find_str(upstream_uri, "?", 1, true)
+    local index = str_find(upstream_uri, "?")
     if index then
         upstream_uri = core.utils.uri_safe_encode(sub_str(upstream_uri, 1, index-1)) ..
                        sub_str(upstream_uri, index)
diff --git a/apisix/ssl/router/radixtree_sni.lua b/apisix/ssl/router/radixtree_sni.lua
index dadbd3dc..ed2c8d65 100644
--- a/apisix/ssl/router/radixtree_sni.lua
+++ b/apisix/ssl/router/radixtree_sni.lua
@@ -22,7 +22,7 @@ local config_util      = require("apisix.core.config_util")
 local ipairs	       = ipairs
 local type             = type
 local error            = error
-local str_find         = string.find
+local str_find         = core.string.find
 local aes              = require "resty.aes"
 local assert           = assert
 local str_gsub         = string.gsub
@@ -218,7 +218,7 @@ function _M.match_and_set(api_ctx)
     if type(api_ctx.matched_sni) == "table" then
         local matched = false
         for _, msni in ipairs(api_ctx.matched_sni) do
-            if sni_rev == msni or not str_find(sni_rev, ".", #msni, true) then
+            if sni_rev == msni or not str_find(sni_rev, ".", #msni) then
                 matched = true
             end
         end
@@ -233,7 +233,7 @@ function _M.match_and_set(api_ctx)
             return false
         end
     else
-        if str_find(sni_rev, ".", #api_ctx.matched_sni, true) then
+        if str_find(sni_rev, ".", #api_ctx.matched_sni) then
             core.log.warn("failed to find any SSL certificate by SNI: ",
                           sni, " matched SNI: ", api_ctx.matched_sni:reverse())
             return false
diff --git a/t/lib/test_admin.lua b/t/lib/test_admin.lua
index 88003616..5663b79d 100644
--- a/t/lib/test_admin.lua
+++ b/t/lib/test_admin.lua
@@ -19,7 +19,7 @@ local json              = require("cjson.safe")
 local core              = require("apisix.core")
 local aes               = require "resty.aes"
 local ngx_encode_base64 = ngx.encode_base64
-local str_find          = string.find
+local str_find          = core.string.find
 local dir_names         = {}