I am Pakistani, want to marry a chinese lady in china, where I can get the single certificate?

If you are in PRC, you should go to the embassy of Pakistan in PRC and ask them to issue a marital status certificate to you.

Or you can apply for the single status certificate at the relevant department in Pakistan, and then have it notarized by the notary office. The certificate should also be authenticated by the Minister of Pakistan Foreign Affairs and Embassy of PRC in Pakistan.