I have visited China three times in last three years. I want one year multiple Visa.

I’m afraid you cannot apply for individual tourist L type. And if you go there for tourism purpose, you can only apply for a group tourist L type. You need to book a tour from a travel agency first. The required documents include your passport and copy of its data page, a notification letter from an authorized unit in PRC, the application form, and an introduction letter from the travel agency.
The consular officers will give you a proper duration and number of entries according to your travel itinerary.