The insulation performance of epoxy resin is a significant constraint to the development of Solid-State Transformers (SST), and bipolar square waves pose new challenges to its insulation properties. This study uses a phase-field model combined with electrothermal coupling and material degradation mechanisms to simulate the dynamic evolution of electrical tree growth. The influence of voltage amplitude, frequency, and rise time on the electrical treeing process is explored. The results indicate that higher frequencies, larger voltage amplitudes, and shorter rise times accelerate the growth of electrical trees, leading to material degradation and insulation failure. This research provides valuable insights for the design of insulation systems in high-frequency electrical equipment, helping to improve the reliability of SSTs and similar technologies.