When a single-phase-to-ground fault occurs in a small current system, the zero-sequence current of each feeder presents complex nonlinearity and nonstationarity, and the signal current at the fault point is very weak, which brings difficulties to fault line selection. However, after fault, the zero sequence high frequency component of fault feeder is larger than that of normal feeder, and the phase is opposite. In this paper, a line selection method combining adaptive notch filter and chaotic oscillator is proposed. By adjusting its parameters, ANF can extract high-frequency signals from feeders in real time and accurately. Chaotic oscillator is more sensitive to the same frequency signals inside and outside, and its scale changes to adapt to high-frequency signal detection of power grid. Firstly, the fifth harmonic of fault zero-sequence current is extracted by ANF, which is used as the input of chaotic oscillator. The obtained system diagram is taken as the fault criterion. Simulation results show that the algorithm is accurate.