Electrolytic DC system is a special ungrounded DC power supply system, because of its high voltage and heavy current and series of low impedance electrolytic cell load, resulting in its ground fault hazard is extremely harmful and not easy to monitor, the traditional monitoring method can not do real-time and multi-point monitoring. In this paper, it is proposed that the detection matrix composed of an active unbalanced bridge is used to select the optimal wavelet basis function to filter and reconstruct the branch current, and the location of the ground fault point is completed according to the amplitude and phase of the processed signal. By constructing a monitoring system and performing filter calculations in an electrolytic DC simulation system with 300 electrolytic cells in series with a voltage of 1.5kV and a current of 400kA, the proposed method is basically consistent with the theory. This method has high detection accuracy, fast response speed, few devices required and low power consumption, which can better suppress the influence of distributed capacitance. Because of its safety and operability, it has certain guiding significance for actual engineering.