Abstract: The signal injection method is widely used for selecting fault line in small-current grounded power system. Because of large fault earth resistance,some mistakes of selecting fault line would occur easily in practice. Based on the analysis of the basic principle and the circulating circuit of the injected signal method, simulation by Matlab, the paper studied the effect of frequency of injected signal on the result of selecting fault line. Then it concluded that decreasing the frequency of injected signal can reduce shunts of non-fault lines, improve application area and accurate rate of the method.