In order to realize high performance for inverter stage of solid state transformer, the small-signal dynamic models of single-phase inverter stage under bipolar and unipolar PWM modulation are built up based on the switching cycle average method and the small signal perturbation method. Consequently, the current to duty cycle transfer function and voltage to current transfer function are deduced respectively. It can be seen that the models and transfer functions are of difference under different modulation method. In order to achieve high control performance, it should be adopted different models when controller are designed. According to the cascade system principle, voltage subsystem and current subsystem are designed and parameter design methods of PI controller are proposed respectively. It has been shown by experiment results that the models established in the paper are corrective. The output voltage of inverter stage has small stable error, high dynamic response and low total harmonic distortion, which means PI regulator and control strategy are practicable and reasonable.