The stability of a grid-tied inverter system will be compromised in weak grid conditions. In order to address this problem, researchers have proposed using a power electronics transformer (PET) to connect inverters and the grid. By adjusting the controller parameters or control strategy, the output impedance of the PET low voltage (LV) converter can be varied to improve the system stability. This paper establishes impedance models of the PET-based grid-tied inverter system and evaluates the stability of the system using the generalized Nyquist criterion. The impact of the variety of control parameters on the system stability is then analyzed. To improve the stability of the system, two stabilized control strategies, namely virtual impedance and digital filter-based methods are proposed. Finally, the effectiveness of these strategies is evaluated through simulations and experiments.