The energy storage-generation inverter system for electric vehicles, based on virtual synchro-nous machine control, will provide reliable support for the stable operation of new energy microgrids. However, the power coupling between the active power and reactive power outputs of the VSG control can lead to steady-state power deviation, dynamic oscillation, and even system instability. This paper presents an electromagnetic dynamic five-order model for the new type of two-stage cascaded three-phase bridge inverter system, which accurately reflects system stability even under varying control parameters. As the limitation of the traditional 'quasi-steady-state' three-order model that only considers the power loop and fails to adequately capture system stability in low inertia and low damping environments, the proposed model enhances accuracy while broadening its applicability. Based on the fifth-order electromagnetic dynamic model, a self-adaptive dynamic virtual impedance decoupling control strategy is proposed in this paper, which takes into account both the system's dynamic and transient processes. By dynamically adjusting the virtual impedance of the system and directly compensating for coupling between active loop and reactive loop, power decoupling of inverter system can be achieved, thereby resolving issues related to poor system stability and control performance caused by strong coupling. The correctness and rationality of the proposed adaptive dynamic virtual impedance decoupling control strategy are ultimately validated through rigorous simulations and experiments.