re: i want to visit china to attend Yarn EXpo from 29-31 March 2012.in Beijing.

Besides the invitation letter, I think you also needs these papers for the China Business visa application:
1. 1. Duly completed Visa Application Form (can be downloaded from this website), affix one recent color photo (30mm X 40mm)
2. Original passport with a validity of more than 6 months and minimum 2 blank visa pages
3. confirmation of invitation from the local foreign affairs office
4. A recommendation letter from your local Chamber of Commerce & Industry or your company Registration
Certificate (photocopy)
5. A supporting letter from your organization;
6. Your tax certificate (photocopy);
7. Your bank statement (personal or company);
8. If it is your first visit to China, your round-trip air tickets and hotel booking
Generally applicants should appear in person with required documents, and may answer questions from the visa officer.