Thank you for the answers.Except the cow fish,I also have 4 other fish in my tank.They are Chrysiptera parasema,Amphiprion nigripes,Chaetodon ulietensis,Paracanthurus hepatus.I find they are all very friendly to each other and there was no aggress condition.
Now I'm trying a new method to make it accept food.I fish out it gently,put
piece of shimp in its mouth,then the cow fish eat it!I've tried it many times,it's successful!
I wish my cow fish can eat by itself activly,god bless......