i would like to stay in china more than one month at a time but if i get one month Visa.

If you get Tourist visa to China, you can extend it before (at a week before) it expires at local PSB within China.

if it is Business Visa then is it possible to extend visa from china..

Usually Bangladesh citizen needs a letter of invitation from China for a smooth application. In the letter the company needs to write the places you will visit. If the visit schedule involves more than one month, you will be given a longer stay in China. Anyway, it's the prerogative of the Chinese Visa Officer.
Wan