I want to attend to the exhibition in china for 7 days.

You should submit your application to the consular department of Chinese embassy in your country. It's located at 120/3B, Vidya Mawatha, Colombo 07.
Tel: +94-11-2676658

Other required documents are:
1. original invitation letter issued by duly authorized unit. It can be acquired with the help of the company who issues you the invitation letter
2. covering letter from your company, including address, telephone number and detailed visiting purpose
3. your company's bank statement, at least recent three months original one.
4. copy of Business Registration Certificate