In traditional control methods of pulse frequency modulation (PFM) there exists a high and variable switching frequency, and stability is poor. The challenge of filter design is multiplied when they are used in APF. Therefore, an improved PFM method based on the principle of an optimal duty cycle is proposed. The optimal duty cycle is obtained by the least squares method. From this, reasonable control parameters are selected. Real-time characteristics of power grid and compensation voltages are introduced into the controller to control the on-time, off-time and hysteresis loop width of the switching device, so as to achieve constant frequency, reduce switching loss, improve control accuracy and dynamic performance. Simulation and experiment of the traditional and the improved methods of PFM verify the effectiveness of the proposed method, and it is easy to design the control circuit with this method. This work is supported by the National Natural Science Foundation of China (No. 61871028).