Photovoltaic inverter system is based on the traditional droop control method to achieve the average distribution of active power, which will cause the maximum power restriction and waste of photovoltaic units. Therefore, in order to improve the optimal utilization rate of photovoltaic unit output energy, an improved droop control strategy for photovoltaic inverter is proposed. The strategy is based on the variable step perturbation observation method to track and control the maximum power of photovoltaic panels. The maximum output power constraint will generate the droop active power compensation, which will be superimposed to the correction of the frequency reference value corresponding to the traditional droop control active power, and keep the voltage amplitude corresponding to the reactive power unchanged, so as to achieve the maximum power output of photovoltaic units in the parallel system. The maximum power constrained droop control strategy of photovoltaic inverter is simulated and verified by PSIM power electronics simulation software. The simulation results show that the variable step disturbance observation method and the improved droop control strategy are effective and feasible in practical application.