Is there any coach from kowloon to Guangzhou airport directly

Well, there is no direct coach that runs between these two places.
So you are suggested to take a taxi from Kowloon Station to Hung Hom Railway Station first. Then take a train to Guangzhou East Railway Station. The train takes around 2 hours on the way.
After arriving, take subway line 3 to get to the airport directly. The duration is about half an hour. It costs CNY 7 per person.