With the influences of global warming and increasing extreme weathers, heavy rain has seriously harmed the safe operation of outer insulation of electric equipment. However, researches on the effect of different test method on AC flashover voltage of porcelain and glass insulators in heavy rain condition are insufficient. In this study, porcelain insulator strings and glass insulator strings were tested in heavy rain with different electrical test methods to obtain their AC flashover voltages. The results show that the flashover voltages from these test methods are different. The average flashover voltage (U_(ave))obtained by the average flashover voltage method is the highest, the 50% flashover voltage(U_(50%))obtained by the constant voltage up and down method is a medium, and the maximum withstand voltage (U_(mv))obtained by the maximum withstand voltage method is the lowest. Moreover, for same kind of insulator strings all AC flashover voltages obtained by different test methods show a negative power exponent relation with rain conductivity. The results from different test methods have intrinsic relations, so it is necessary to select appropriate test method according to specific testing circumstance.