There seems no direct bus to Taishan. You can take the airport express bus line 4 and get off at Guangyuan Bus Station. Then take a taxi to Taishan as it is only about 3km from the station to Taishan.

Alan, I think you gave me wrong info. I am going to Taishan (south of Jiangmen), and if I can't go from Baiyun airport, which bus station in Guangzhou that provides express bus to Taishan?

Ah,I am really sorry for that. I have typed wrong numbers.
You can go Guangdong Provincial Coach Terminus opposite Guangzhou Railway Station. There is express bus to Taishan every half a hour. Liuhua Bus Station nearby also have express buses to Taishan. It takes about 2 hours on the way and the ticket price is about 30RMB.