I have to visit Beijing on official visit from United Nation from 15 Dec to 18 Dec 2015.

Since you said you will be on an official visit to China, may I know what passport do you hold? If it is an official one, then you don't need any entry permit. If it's an ordinary one, you need to apply for a W type. The required documents include a properly filled application form, a recent bare-headed color photo, your passport, a diplomatic note issued by the UN, and an invitation letter from PRC government. After you have collected all the required documents, submit them to the embassy and wait for the approval result.