The power electronic transformer, based on modern power devices, achieves multi-stage power conversion and energy management, playing a significant role in the new power system. Its insulation material, polyimide, is prone to partial discharge (PD) under high-frequency electrical stress, leading to accelerated insulation aging. To investigate the influence mechanism of high-frequency voltage frequency and amplitude on PD characteristics, a high-frequency partial discharge experimental platform was established to study the variations in PD parameters under different voltage frequencies (10 kHz–40 kHz) and amplitudes (2 kV–4 kV). The experimental results indicate that the discharge current amplitude is proportional to the applied voltage amplitude, and with an increase in voltage frequency, the discharge current amplitude shows an initial increase followed by a decrease, forming a “turning point” phenomenon. Considering the trend of polyimide's dielectric constant decreasing with increasing voltage frequency, a modified plasma PD numerical model was constructed to describe the spatiotemporal evolution of particles during PD and the generation pattern of the oxidative byproduct, ozone. The analysis of the impact of applied voltage amplitude and frequency on the discharge current amplitude revealed that the mismatch between the charge dissipation rate due to the dielectric constant and the applied voltage frequency is the reason for the “turning point” in discharge current amplitude observed at higher voltage frequencies.