High-frequency transformers generally operate under high-frequency nonsinusoidal voltages, and their epoxy resin insulation needs to withstand rapid reversal of voltage polarity and high dv/dt. In this case, epoxy is more prone to degradation and failure compared to power frequency sinusoidal and DC voltages. Along with the local temperature rise in high-frequency transformers, the combined effect of high temperature and electric field will cause rapid aging and even the breakdown of epoxy. To analyze the influence of temperature and frequency on the high-frequency breakdown characteristics of epoxy, this paper takes epoxy for high-frequency transformers as the research object and tests the breakdown strength of epoxy at different frequencies and temperatures under bipolar square wave voltage. The test results show that the breakdown strength decreases with increasing frequency and shows a trend of first increasing and then decreasing with increasing temperature. We established a breakdown model for the variation of free volume with frequency under high frequency, and believe that the overall free volume of the epoxy increases with increasing frequency, leading to a decrease in the high-frequency breakdown strength. Based on the thermal breakdown model, we analyzed the effect of temperature on the short-term breakdown characteristics under high-frequency electric fields. We found that the breakdown process below the transition temperature is affected by thermal equilibrium, while the breakdown process above the transition temperature is closely related to the free volume change caused by glass transition.