In this paper, two kinds of photovoltaic cell’s mathematical model are presented, and their curves of output characteristic are drawn. The MATLAB/SIMULINK model of PV power generation system is based on PV cell's physical equivalent circuit, its mathematical model 2 and PV module YL 120 P-17b/1470×680. Due to the single peak of the output U-P characteristic, PV module has a Maximum Power Point (MPP). The fixed-step and variable-step duty ratio perturbation methods are applied to the model of PV power generation system that has been established for MPPT simulation research in this paper. In addition, the advantage of variable-step duty ratio perturbation method and the important role of the storage battery have been proved by the simulation results.