How to get China visa.I am sri Lankan .now I work in south korea .

If you want to apply for a tourist L type, you need to prepare passport and copy of its data page, the completed application form attached with a passport-style photo, residence permit in Korea, an invitation letter from China tourism bureau, travel itinerary, round trip air tickets and recent 3-month bank statement, as well as other documents the consular officers consider necessary.
Next, go to the CVASC to submit the application.

Hi can you tell me how to get invitation letter

You are advised to find an agency to help you get that. Or you can book a tour from a travel agency, and then the travel agency may be able to get the invitation for you.