The virtual synchronous generator (VSG) technology is an effective method to solve the problem of decreasing system inertia and damping caused by increasing the permeability of new energy source with power electronic converter as the interface.But the parameters of the virtual rotational inertia, the core parameter, have been lack of the selection principle. Especially in the case of multiple parallel machines and more variable parameter control strategies, the improper selection of parameters can easily cause system oscillation. Take a two-machine system as an example,with the help of the high-order small signal model and its analysis method,the virtual rotary inertia is studied in detail. The selection principle of virtual moment of inertia for inverter with synchronous generator characteristics is proposed.which selects the parameters according to the stability threshold and system operation requirements.It can improve the stability performance of the system, and the correctness of the analysis is verified by simulation and experiment.