You mean the Z visa of China? You could just apply it from the Chinese embasssy in Pakistan with the following documents:
1. Original Visa Notice Sheet from an authorized governmental apparatus /organization in China;

2. Alien Employment License of the P. R. China /Working Permit by the State Administration of Foreign Experts Affairs, P.R. China /Letter of Invitation for Foreigners Engaged in Offshore Petroleum Operations in the P.R. China/Registration Certificate of Foreign Enterprises Permanent Office in China

3. Physical Examination Record for Foreigner (the blank form can be downloaded from http://pk.chineseembassy.org/eng/lsfw/) and attested by Ministry of Foreign Affairs of Pakistan

4. Original passport with validity more than six months and minimum 2 blank visa pages;

5. Photocopy of the first 2 pages (personal information) of the passport and the recent Chinese visa page if there is any;

6. Fully completed Visa Application Form in Chinese or English, attached with one of the applicant's recent color photograph;

7. Employment Contract

8. The copy of your Employer's Company Registration Certificate in China.