I am invited as speaker in conference. I have got an invitation. How can I apply for visa?frm Dharan

You need to apply for a business visa to China. And visa application could be delivered to the Consular Section of the Embassy either by yourself or the entrusted travel agencies. Visa application by mail is not accepted. Required documents are your passport, application form with photo, Visa recommendation letter from your employing company, organization, etc.
Sometimes, you need to provide other additional documents according to the merits of each applicant, e.g. bank statements, round-trip air tickets booking, etc.