In order to reduce the maintenance scope and quickly locate the fault line of the distribution network，a method of representing and processing distribution line segment information by the structure of the tree and graph is put forward. By analyzing the mechanism of single-phase-to-earth fault in the ungrounded neutral system，the fault segment location method and the reasoning-rules determining method are given，with the fault segment lines explicitly pointed out. Numerical simulation results show that the proposed location method can solve the on-line fault segment location problem of single-phase-to-earth in the ungrounded neutral system.