Abstract: In recent years, with the rapid development of China's rail transit industry and the expansion of high-speed rail lines, voltage insulation issues faced by EMUs operating in harsh environments have become increasingly prominent. This paper establishes a three-dimensional model of the high-voltage box using SolidWorks, imports it into COMSOL for electro-thermal coupled simulation, and investigates the distribution characteristics of electric fields and temperature fields within the high-voltage box. Simulations analyzed the distribution patterns of electric fields and temperature fields within the high-voltage box under both operating voltage and overvoltage surge conditions. The study focused on examining electric field concentration phenomena and temperature rise characteristics in insulation weak points and high-risk areas. Results indicate that the electric field distribution within the high-voltage box exhibits regional characteristics, while temperature distribution is significantly influenced by current and electric field effects.