With the increasing numbers of power electronic devices in system, virtual synchronous generator(VSG), which mimics the motion of rotational synchronous generators(RSGs), is proposed to cope with the revolution in system structure. However, there lacks precise power angle transient descriptions, leading to inaccurate stability analysis in VSGs. In this paper, analytical methods for perturbation systems are introduced in VSGs to solve for power angel transient waveforms. The reactive power control loop is first considered and voltage vibration is described by power angle afterwards. On the base of that, K-B method is introduced and optimized to calculate power angle trajectories in VSGs. Moreover, second-order KBM method is implemented for more precise dynamic descriptions. Simulations are finally executed to validate the accuracy of analytical solutions.