Active power oscillations and frequency fluctuations appear simultaneously in virtual synchronous generator (VSG) systems with fixed rotor inertia and damping coefficients. Thus, an optimisation control strategy to self-adjust the parameters is proposed in this study to suppress the power oscillations and frequency fluctuations. A small-signal model of parallel VSG systems is established, and the impact of rotor inertia and damping coefficients on the output active power is analysed. Then, the parameter optimisation control for the self-adjusting inertia and damping coefficients is introduced according to the rate of change and the deviation of virtual rotor electrical angular velocity. Notably, the changes and deviations are limited in a range to ensure the dynamic and steady-state performance of the system. Based on the principle of the equivalent synchronous generator, a virtual inertia matching method for parallel VSG systems is accordingly obtained. The feasibility of the proposed control method is verified by extensive simulations. The results confirm that this method can not only suppress the power oscillation in the transient connection of the VSGs to the AC bus, but also improve the frequency support. In turn, it ensures that the load power is proportionally distributed among the VSGs.