"My English is poor" "My English is awful" "My English is not-so-good" "My English is terrible" "My English is dire" "My English sucks" "My English Blows" "My English is lamentable" "My English is a bit patchy" "My English isn't as good as I would like it to be" "I don't speak English very well" "I wish my English were better" "I need to work on my English" "Please excuse my English" etc. I bet it's better than you make out though! You can always add: "...but it's better than your Mandarin!"

ok,you are right..I want to sak how to say my English sucks.But I see clearly now

Sorry, it's not clear what you're asking. If you're asking if "My English is poor" is correct, the answer is yes.