A grid-connected inverter (GCI) based on a virtual synchronous generator (VSG) control may cause overshoot or oscillation of output power and frequency under fluctuation conditions. A VSG control method based on an inertial adaptive algorithm is proposed to improve the dynamic performance of a GCI. This method directly analyzes the power angle and output characteristic curves of a GCI based on VSG control, and deduces the relationship between the output power and the frequency change rate. By judging the four acceleration and deceleration operation intervals of the inverter, the deviation between the virtual output active power and reference power is employed to avoid dependence on the change rate of the output frequency. With an adaptive control algorithm, the parameters can be adjusted continuously and smoothly, so that the frequency and output power fluctuations of the GCI can be restrained. Compared with existing methods, the proposed method does not require frequency differentiation directly. This avoids the introduction of system noise. It can also achieve a wider range of continuous adjustment of inertial parameters. Simulation results verify the correctness and effectiveness of the proposed method.