A study was conducted on the electrothermal coupling characteristics of epoxy resin-cast insulated busbars, aiming to enhance their operational reliability and performance through optimized design and multi-physics field analysis. This paper established an electrothermal coupling mathematical model incorporating electromagnetic and temperature fields, and utilized the finite element method to simulate and analyze the electric field distribution and temperature rise characteristics of epoxy resin-cast insulated busbars. Through three-dimensional solid modeling and mesh partitioning, the uniform distribution pattern of the electric field under the influence of the semiconductor layer was revealed, and it was found that the maximum temperature on the outer shell surface reached 90 °C during long-term operation. The model and analysis methods established in this paper provide theoretical basis and practical references for the engineering design and optimization of cast-type insulated busbars.