When the fault occurs in the small current system, the characteristics of zero sequence current of each line show complex nonlinearity and nonstationarity, and the current at the fault point is very small, which brings some difficulties to the fault line selection. This paper presents an improved detection method of PLL and chaotic oscillator. The improved PLL can not only accurately track the signal of specific frequency, but also do not need to transform and transform. The chaotic vibration subsystem has a high sensitivity to the signal with the same frequency as the internal driving force, and the phase diagram of the Duffing system is different from the external signal at the same time. First, the fifth harmonic in the zero-sequence current is locked by the improved phase-locked loop, and the signal is used as the signal to be measured in the chaotic system. The signal is detected by the Duffing oscillator system. The state of phase diagram of the signal to be tested by the chaotic system is inconsistent with that of the normal line and the fault line, and the fault line can be judged from the state of the phase diagram. Different fault conditions are simulated to verify the reliability of the algorithm.