Abstract: The vehicle roof-top high-voltage box is a key component for city EMU. It is important to know the electric field distribution characteristics of the high-voltage box for the safe operation of city EMU. The working principle and structural composition of EMU high-voltage box are expounded, a three-dimensional finite element electric field calculation model is established, and the potential and electric field distribution of high-voltage box and internal devices are simulated and analyzed. According to the simulation calculation results, the high-voltage box internal electric field distribution mainly concentrates on high-voltage device conductive metal, joint spot of insulation material and air, high-voltage cable end, vacuum interrupter upper, bottom and head parts, while the electric field strength at lightning arrester head is relatively stronger. It is proposed to carry out chamfer treatment on the conductive metal contacting insulation material and to add insulation protection on conductive metal at ends.