i'm a holder of F visa valid for 120 days.can i work in china for just 90 days?

Hi Mori, please read your visa carefully. I am wondering if you mean visa validity or visa duration by saying F visa valid for 120 days. BTW, F visa holders are not allowed to work in China, I think what you mean is to do business for 90 days, right? if yes, then it is ok.