Selecting single-phase ground faults in small current systems is difficult. To tackle this, first the reason why the existing line selection methods cannot effectively detect and remove the fault phase is analyzed after on-site investigation. The characteristics of the single-phase grounding fault and the principle of line selection of a small current grounding system are analyzed in depth. The method of grounding line selection for small current is described. On this basis, the principle of grounding protection based on a half-wave Fourier algorithm is proposed. An action criterion algorithm based on a half-wave Fourier algorithm is established. Given the action criterion, a single-phase ground fault line selection system for small current grounding system is designed, and the hardware part of the system is selected. This work is supported by National Natural Science Foundation of China (No. 5187070061).