Abstract: Secondary frequency regulation can ensure that the power system always meets the requirements of frequency stability during the primary frequency regulation process. The grid-connected inverter using the virtual synchronous generator（VSG）control strategy only has the characteristics of primary frequency modulation when operating off the grid. If the output frequency of the VSG exceeds the safe operating range,the stable operation of the microgrid will be affected. Therefore,it is necessary to add a fixed integral coefficient to the active control loop to realize the secondary frequency modulation of frequency deviation-free control. However,traditional secondary frequency modulation has an inherent contradiction between recovery time and frequency oscillation. An improved VSG secondary frequency modulation strategy based on transient derivative polarity subsection optimization is proposed in this paper. This strategy divides the frequency recovery process into two stages based on the response characteristics of frequency recovery,and configures different integration coefficients for the two stages,so that the entire frequency recovery process has a faster recovery time and avoids frequency oscillations. It also provides a basis for setting the key parameters of the proposed method. Finally,the correctness and effectiveness of the improved control strategy proposed in this paper are verified by simulation and the hardware-in-the-loop platform based on HIL.