my wife is at china now on study visa,he is feeling homesickness due to which she wants to call me

If you plan to visit your wife, you should apply or a S type. If she has residence permit, you may apply for S1 type. Otherwise, you need to apply for S2 type. But I think it's easier for you to apply for a S2 type. S1 type requires the invitation letter from her school. I am not sure if her school is willing to issue an invitation letter. By the way, you need to go through the medical examination at the appointed state-owned hospital in your country and then get the paper authenticated by Ministry of Foreign Affairs of Pakistan. The S2 type doesn't require your medical examination report. The same materials for S1 and S2 are: your passport, filled application form, your recent passport photo, invitation letter from your wife and your marriage certificate.

S1 allows you to stay longer than 180 days. S2 type just allows you to stay as long as 180 days.