May I visit China when I am coming back from Japan?

if you can get a valid China visa, you can go to Beijing to stay for one week after visiting Japan. You'd better obtain a China visa in Tehran if you live in Iran or Iranian citizen.

The required documents to get a China tourist visa in Tehran are:
1. your passport
2. one completed visa application form V.2011A
3. one passport photo
4. the round-trip flight tickets
5. hotel bookings in Beijing

However, sorry to tell you that China embassy in Tehran maybe doesn't accept individual tourist visa application, if the application is denied, you can go to Japan to try to apply China visa.