Due to the instability of renewable energy generation and the diversity of loads, the LLC-based power electronic transformer (PET) is susceptible to input and output disturbances, thus leading to oscillations. Firstly, by analyzing the small-signal model of the LLC converter, the oscillation mechanism with the classical voltage-mode control (CVMC) is revealed. As the input voltage decreases or the load increases, the operating frequency of the LLC converter decreases and thus operates in the under-resonant state, which results in a decrease in phase margin. To address the problem, an inertial voltage-mode control (IVMC) strategy is proposed in this paper. Compared to the CVMC strategy, the proposed IVMC strategy has better stability against input and output disturbances. Finally, the correctness of the aforementioned theory is verified by simulation results.