In order to improve the output efficiency of PV generation system, an effective maximum power point tracking method of PV array is necessary. And the perturbation and observation (P O) method is most widely used due to its simplicity and ease of implementation. But there is oscillation around the maximum power point caused by P O, so this paper proposed an improved variable step size perturbation and observation (P O) method. The method introduced power factor P in step change trigger conditions and step form. Variable step regional changed little when the external environment occurs a substantial change, so MPPT can keep the good dynamic and steady state accuracy under different external conditions. It improved the problem that traditional variable step size P O method can’t take into account dynamic and steady state accuracy when the external environment occurs a substantial change. Compared before and after the improvement of the two methods in the simulation and experiment, the results show that the improved variable step size P O method is effective.