To suppress the second harmonic current in the front-stage circuit of a two-stage single-phase photovoltaic grid-connected inverter, the causes of the second harmonic current in the front-stage circuit are analyzed using a switching period average model, and based on this an improved modulation method for suppressing the second harmonic current is proposed. This method corrects the duty ratio according to the bus voltage obtained by real-time sampling. This is easy to realize without complicated processing. The small signal transfer function of the front-stage Boost circuit using an improved modulation method is analyzed, and a voltage tracking method of the maximum power point of photovoltaic cells based on a differential ahead PID control algorithm in an under-damped condition is proposed. The simulation results show that the improved two-stage single-phase photovoltaic grid-connected inverter has little oscillation in the maximum power point tracking process, the output current of photovoltaic cells is stable, and the second harmonic current suppression effect is remarkable.