Abstract: The fifth harmonic phase of fault line and health lines is approximately opposite， when coal mine Electric Network occurs Single-phase-to-ground Fault。 But the amplitude of fifth harmonic is too small to select directly the fault line。 Duffing chaotic system is Sensitive to the specific small signal and has the immunity of noise。Based on the characteristics ，a approach of detecting faulty line of compensated Electric Network is proposed。The approach is named Duffing dichotomy 。Duffing system obtain the input signal by the EMD method. Finally，The principle to select is simulated with Matlab/Simulink and the simulation results are analyzed，which show the approach is effective。