Power angle stability is an important prerequisite for virtual synchronous generator in grid stability. However, in the event of a voltage transient fault on the grid side, the power angle tends to increase continuously causing destabilization due to the unbalance of the torque. In this paper, through the comparative analysis of Q-U controller, it is concluded that the use of constant voltage control has an optimal effect on the power angle stability. Based on this, a virtual capacitor-based VSG transient power angle compensation strategy is proposed, where a new ground capacitor branch is added at the PCC point to increase the virtual electromotive force. Unlike the traditional improvement in P-f controller, this paper reduces the continuous increase of the power angle through the modification in Q-U controller, and verifies the validity of the proposed control method through experiments.