When single-phase earth fault occurs in resonant earthed system, there are significant differences in spectrum distribution of fault phase current between fault line and non-fault line. Approximate entropy has its unique advantage in the non-stationary signals analysis as a way of measuring the complexity of time series. Then the possibility and applicability of using approximate entropy (ApEn) as the criteria in resonant earthed system fault line detection is discussed. The fault model is built on the basis of Matlab. Fault conditions such as fault inception angle, grounding resistance and fault position are considered. Some factors such as noise, asynchronous sampling and different compensation degrees are also taken into account. Simulation results verify the feasibility of ApEn criterion for fault line detection.