Abstract: In order to improve the output efficiency of PV generation system, A novel variable step size perturbation and observation(P&O) method is proposed to track the maximum power point of PV system. Based on the mathematical model of PV system, this method tracks the maximum power point by regulating the output voltage after measuring the changes of its output power. The simulation model of PV system is established, and the experiment is implemented. The experimental results show that the method can track the maximum power point fast and exactly. Therefore adaptive P&O has better steady-state and dynamic performance than the traditional P&O, and can improve the efficiency of photovoltaic power generation system effectively.