Virtual synchronous generator (VSG) is a widely used grid?forming control strategy. However, due to the nonlinear power angle relationship caused by the analog of external characteristics of synchronous generators, VSG is similar to synchronous generators and is prone to lose synchronization with the power grid under severe disturbances. This paper analyzes the large signal power angle stability of grid?connected VSG, and an additional damping method to improve the power angle stability is proposed, the method is a theoretical analysis method combining the linearized model with the nonlinearized model. Firstly, the linearized model is used to qualitatively analyze the damping ratio and the rate of change of frequency (RoCoF), which shows that the additional damping method can improve the power angle stability without reducing the frequency stability. On this basis, the nonlinear model is used for quantitative analysis to accurately evaluate the power angle stability of VSG after a power grid fault. Theoretical analysis and simulation show that adding additional damping into the active power control loop can improve the power angle stability of VSG.