For extending the longevity and improving the efficiency on power generation of photovoltaic cells , a variable step algorithm based on constant voltage and hysteresis loop control method is proposed and the voltage disturbance step value in this article is determined according to the analysis to the influence of light intensity for photovoltaic cells and the power characteristics of near maximum power point. By the block of MATLAB and the data of the experiment ,the simulation model of photovoltaic cells and variable step MPPT are built in the article and the strategy is verified. The simulation results and the testing data show that the algorithm can track power output of photovoltaic cells in real time well despite the fast changes of light, and can effectively suppress oscillations near the MPP, which has good dynamic and steady-state characteristics and confirms the validity and correctness of the algorithm.