The invention relates to a determining method for a power distribution single-phase earth fault contingency set. The method includes the following steps that step1, a power distribution network equivalent circuit diagram is established; step2, the starting end voltage of a feeder line section and the current which flows through the section are calculated; step3, an impedance method is used for performing a fault diagnosis on the feeder line section, measurement impedance at the fault moment is obtained through calculation by the utilization of voltages and currents before and after a fault at the starting end of the feeder line section and then compared with unit impedance of the feeder line section to obtain a possible fault distance, and therefore all feeder line sections within the fault distance are possible fault sections; step4, the step2 and the step3 are repeated, a fault diagnosis is performed on other feeder line sections until all the feeder line sections are judged, and the set of all the possible fault sections is the contingency set of the fault. According to the method, the fault diagnosis is independently performed on each feeder line section, so that errors, which are generated because many branch circuits and single-phase lines of a power distribution network are arranged, of the impedance method are eliminated, and the accuracy of the fault diagnosis of the power distribution network is improved.