Abstract: A fault line selection method based on morphological wavelet decomposition is proposed for the single-phase grounding fault in small current grounding systems. Firstly, the zero-sequence current of each line is preprocessed by the morphological gradient algorithm. Then, the morphological Haar wavelet is used as the core processor to perform multi-scale morphological wavelet decomposition on the zero-sequence current, collecting and calculating the approximate components and detail components for the extraction of fault transient features. Next, the modulus maxima and their polarities of the detail coefficients at each scale are calculated. The fault line selection is achieved by combining the modulus maxima intensity and polarity features. The MATLAB/Simulink simulation verification shows that this method can accurately detect the single-phase grounding fault line.