Can I apply again to get visa .do they give me the entry permit or the will refuse to give?

If your current Chinese visa is valid, you don't need to apply for another one to enter China to go to the canton fair. Otherwise, you are supposed to re-apply for it.

Thanks William .i would like to tell you the entry permit has been expired .I know I need to apply again but let me know how is the possibility to get visa on canton fair invitation

Generally speaking, chances are good for you to get a Chinese visa as long as you have all requirements including the invitation letter from the canton fair. But I cannot guarantee that. You are advised to inquire the China embassy to ask if the duly authorized invitation letter is needed for the application.