Rainfall is considered to be one of key factors affecting safe operation of electrical equipment of power transmission line and seriously threatening reliability of power supply. In this paper

the corona discharge characteristics of grading ring of 220 kV insulator are studied under power frequency AC voltage in dry and rainfall conditions.Statistics and comparative analysis of the discharge inception voltage

discharge repetition rate and the mean discharge in the two conditions are performed. At the same time

the discharge ultraviolet images under two conditions are taken and the variation rule of discharge photon number with the applied voltage is statistically summarized. A finite element simulation model is set up to simulate and calculate the surface electric field of the grading ring under different conditions. The test results show that the discharge inception voltage at rainfall condition is lower than that in dry condition

the distribution range of the discharge PRPD spectrum is wider and the discharge repetition rate and average discharge are larger. With the increase of applied voltage

the area of the discharge spot on surface of the grading ring gradually increases and the spot area at the discharge position at rain conditions is larger than the discharge area in dry condition. The simulation results show that the condensation of rainwater on the surface of the grading ring causes local electric field distortion

which keeps the inception voltage of the surface of the grading ring decrease. The research can deepen the understanding of the influence mechanism of the rainfall environment on the insulation performance of electrical equipment.