My wife is a China citizen.My baby and I plan a family visit there about 2 mths,what document need?

You can apply for family visit visa from the Chinese embassy in your country. Its validity is 30 days. When you enter China, you can extend it for six months or one year. The documents you need are original passport with at least two blank pages, completed visa application form with a recently-taken passport photograph affix on it and invitation letter from your wife.

In order to extend your visa successfully, you may need to prepare extra documents. Please read this page:
https://www.travelchinaguide.com/embassy/visa/family-visit-permit.htm