The reliability and security of DC system have a significant impact on normal operation of various load devices in power stations and substations. It is also closely to production safety grid. As one of the detection methods in DC system grounding fault, the low frequency signal injection is easily influenced by the capacitance to ground and influence the accuracy of detection. According to this problem, deeply analysis the method, principle and existing problems of low frequency signal injection, put forward in the low frequency signal injection method based on wavelet transform, selecting the suitable wavelet function, and then extract and process the signal. The simulation result shows that the method can overcome the influence of the capacitance to ground and in order to locate in grounding fault accurately.