The microgrid inverter with VSG control strategy has only primary frequency modulation characteristic in off-grid operation mode. Therefore, it is necessary to introduce secondary frequency modulation control. This paper finds that the traditional secondary frequency modulation of VSG adopts a fixed integral coefficient, and there is a contradiction between the recovery time and the frequency oscillation through theoretical analysis. Aiming at this problem, this paper proposes an improved VSG secondary frequency modulation strategy with variable integral coefficient. According to the response characteristics of frequency recovery, the strategy divides the frequency recovery process into two stages, and configures different integral coefficients for the two stages, so that the entire frequency recovery process has a faster recovery time and avoids frequency oscillation. Thus, the rapidity and stability of the entire frequency recovery process of the VSG are improved. Finally, the correctness and effectiveness of the proposed improved control strategy are verified by simulation and experimentation based on hardware-in-the-loop platform.