In order to accurately analyze the dynamic characteristics of Power Electronic Transformers (PET) under large signal disturbances, a new rotator modeling method based on power balance is proposed. This method is used to model the PET with rated capacity of 3 MW and compared with the traditional switch model. The simulation results show that the proposed model has validity and practicality. It can not only accurately reflect the behavior of the system under the disturbance of large signal, but also can reduce the simulation time. In order to realize the decoupling control of the front and rear DC bus voltage, a voltage feedforward compensation strategy is proposed. On this basis, a uniform time domain analytic equation of bus voltage is established, the influence of system parameters on DC bus voltage under large signal perturbation is quantitatively analyzed, and the relation curve is given, which provides the criterion for the design of each module controller. This work is supported by National Natural Science Foundation of China (No. 51567009), Natural Science Foundation of Jiangxi Province (No. 20181BAB206033) and Cultivate Plan for Application Research of Jiangxi Province (No. 20181BBE58008).