When the single-phase to ground short-circuit occurs in the small-current grounding system of the active distribution network, through the theoretical analysis of single DG access to distribution network does not affect the zero sequence current conclusion, so a discrete orthogonal S-Transform (DOST) Fault location method of zero sequence current energy similarity was proposed. Firstly, discrete orthogonal S transform is applied to the transient zero sequence current signal uploaded by the detection equipment on the line, and then the energy similarity of detection points at both ends of the line section is calculated by using the data in the time-frequency matrix obtained, the energy similarity is less than the setting The threshold is the fault interval. The simulation results of Matlab / Simulink show that the method is accurate and effective and can be applied to the fault zone location of active distribution network.