Abstract: It is of great significance to study the electric field distribution characteristics of post insulators under pollution conditions for the safe and stable operation of EMU .In this paper, the FQJG2-30-16-400-M model roof composite pillar insulator is taken as the research object, and the following research is carried out: the surface of the insulator is subjected to rain test, and 40kV voltage is applied. Flashover occurs first in the skirt close to the gold fittings, and the dry band appears. When the skirt edge forms large particle size of water droplets, partial discharge is likely to occur. Using finite element analysis software comsol in insulator with the near umbrella skirt set dry band and water surface, the surface of the sheath dry band exists, the electric field distortion, stem diameter in the dry band field intensity, the largest on the surface with dry field strength is greater than the surface dry field, dry with a maximum field strength as dry with increase in the number decreases. When water beads appear on the surface of an insulator, the field intensity near the beads increases significantly, and the closer to the central axis the beads are, the greater the electric field intensity is. The maximum electric field intensity of water droplets near the high pressure end is greatly affected by the radius, while that near the low pressure end is not obvious.