diff --git a/lib/loader/egg_loader.js b/lib/loader/egg_loader.js
index <HASH>..<HASH> 100644
--- a/lib/loader/egg_loader.js
+++ b/lib/loader/egg_loader.js
@@ -95,6 +95,7 @@ class EggLoader {
       if (process.env.NODE_ENV === 'test') {
         serverEnv = 'unittest';
       } else if (process.env.NODE_ENV === 'production') {
+        console.warn('NODE_ENV is production but EGG_SERVER_ENV is empty, forgot `EGG_SERVER_ENV=prod`? see https://github.com/eggjs/egg-core/blob/master/lib/loader/egg_loader.js#L82 for more detail.');
         serverEnv = 'default';
       } else {
         serverEnv = 'local';
feat: warning when missing EGG_SERVER_ENV at production (#<I>)