How can I apply for China work visa? What are the requirements?

You mean Chinese Work visa? Then the requirements are:
1.Applicant's passport with blank pages and at least 6 months validity left before expiration.
2. A truly and completely filled application form affixed with one passport-size or 2x2 colored photo, with white background, full-face front view, and no hat. Photo must be glued on the application form. Stapled pictures will not be accepted. Scanned photos will also not be accepted.

U.S. passport holders must submit two application forms with two photos.

3. An original Visa Notification Form issued by Chinese authorized unit.
4. Original Working Permits, such as: Working Permit issued to foreign experts by The State Administration of Foreign Experts Affairs, Alien Employment License of the People's Republic of China issued by Ministry of Labor and Social Security, Letter of Invitation for Foreigners Engaged in Offshore Petroleum Operations in the People's Republic of China, etc.

For those engaged in offshore petroleum operations, also provide a guarantee letter from your shipping company.

5. All working visa applicants shall provide a Physical Examination Certificate for Foreign Citizen (physical exam forms, guidelines and list of accredited hospitals are available at the visa counter).
6. All Philippine passport holders who will go to China for the first time must submit an original NBI clearance valid for travel abroad.
7. The emergency contact information page in the applicant's passport should be filled out and photocopied.