The invention discloses a segmenting method for a subway contact net lead in a CDEGS lightning model. The segmenting method is characterized in that a least value N of the number of contact net lead segments is calculated through a model, namely N=(m+1)*(2int[(25a-183)/366]+3), wherein m is the number of span, a is the average span, and int[] is an integral function. The segmenting method aims at a subway system at a viaduct segment. An HIFRIQ module of CDEGS software is used for modeling the power supply mode of the subway contact net at the viaduct segment. The invention provides a conductor segmenting method for calculating an insulator withstand voltage of the subway line at the viaduct segment based on a CDEGS simulation model, wherein a requirement for high calculation precision is satisfied and calculating efficiency is improved.