A quantum mechanics model used to describe a n-photon transition within an atomic system is presented, and a semiclassical approach is used to obtain Maxwell-Bloeh equations so as to describe the multi-photon optical bistability system. From these equations the conclusions can be easily derived as follows: under stationary conditions, the flourescence intensity of multi-photon optical bistability is propotional to N orN - 1/(2n-1)in the cooperative region and is propotional to N in the single atom region.