In Photovoltaic (PV) system, the output characteristic of PV cell is susceptible to external environment, such as temperature, light radiation etc, and showing obvious nonlinear. Therefore, in order to improve the efficiency of PV system, the maximum power point tracking (MPPT) of PV cell is essential. This paper mainly introduces the simulation of PV cell and the methods commonly used in Maximum Power Point Tracking of PV system. For the perturbation and observation (P&O) method the paper presents a new variable step size algorithm and builds a simulation model. The result shows that the proposed algorithm has a better effect on maximum power point tracking of PV cell.