Abstract: Based on the beam interferential theory of Fabry-Perot semiconductor laser,a bistable model of vertical cavity semiconductor optical amplifier (VCSOA) was established. According to recently experimental parameters,the bistable characteristics of VCSOA are investigated by simulation and three methods of controlling the width of hysteresis loop have been achieved:to enhance the bias current properly,increase top mirror reflectivity of VCSOA without changing the ratio of bias current versus threshold current or decrease initial phase detuning.All the methods can increase the loop width of the hysteresis loop.