Hausdorff Distance is introduced into Fault line selection in a small current grounding system. To solve the problem of fault line selection, this paper proposes an improved Compositive Hausdorff Distance, which uses similarity measurement based on transient zero-sequence currents signals as criteria, to calculate waveform similarity of transient zero-sequence currents. By using MATLAB/SIMULINK software to simulate different fault conditions, the results show that Compositive Hausdorff Distance has the advantages of high accuracy,strong noise immunity, fast detection speed and low limit of sampling frequency, which are not affected by grounding resistance, fault voltage initial angle, short circuit distance and power grid structure.