how can I get a F or M visa, which duration is 90 days?

Cassandra, it may be helpful. But it isn't a guarantee that you will get a 90 days F/M type. You know, the consular officers have the final say. By the way, you can extend it at entry and exit management bureau in Shanghai even you fail to get a 9 months one.

but it seems now from Shanghai Exit and Entry Bureau, very difficult to extend it.
for L type, cannot extend. For F and M type only those who hold the China Government Invitation Leter can submit the documents to extend it. But I've called to them 021-28951900, the policeman told me "you can submit the documents, but not guarantee you will sucessfully get the extention."

actually very worried what should I do...

If you can't extend it, you have to leave China and apply for another F/M type at nearby countries. Another solution you may have a try is to apply for a double or multiple entry M/F type valid for half a year.

thanks so much