A method for evaluating the performance of lines in a DC power grid includes: in a plurality of operating modes, segmenting a plurality of lines; setting a ground fault for each segment point on the plurality of lines respectively; and for subsequent fault clearing, obtaining the overvoltage value of multiple fault occurrences at each segment point; counting the frequency of each overvoltage between the minimum and the maximum overvoltage levels according to the voltage level; multiplying the frequency of each voltage with the occurrence probability of each operating mode and adding the multiplied results to obtain the total overvoltage frequency of each segment point of the multiple lines; and integrating the total overvoltage frequency with the corresponding discharge probability of the typical air gap of the corresponding line to obtain the flashover probability of the typical air gap in the multiple lines. According to the multi-step span included in each line, the total flash of the multiple lines in the DC network is calculated; and if the overall flashover rate is greater than the expected flashover rate, the air gap length is increased.