my wife is on x visa in china. what will b the procedure for her to send me invitation for s1?

She has to provide the copies of her resident permit and passport info page apart from the invitation letter. Also you should provide the kinship certificate, marriage certificate for example. The invitation letter should include:
1. Your name, gender and date of birth.
2. why you come to PRC, when you arrive, where to live, how long you stay, relationship with the inviter and financial resources.
3. your wife's name, tel, address and signature.

Also, you should provide the health certificate. The form can be download from the website of Chinese embassy in Pakistan. You should take the examinations at local state owned hospital and get it authenticated by the Minsitry of Foreign Affairs of Pakistan.