The invention relates to the field of power systems and the automation of the power systems, in particular to a method for single-phase earth fault line selection of a small current grounding system. According to the method, the zero sequence circuit of a single-phase earth fault is analyzed, the frequency characteristic of the transient state characteristic quantity of the fault is obtained, an optimal FIR filter is designed according to the frequency characteristic to extract the components of transient state capacitive zero sequence currents with obvious fault characteristics, and the interference of noise and the interference of the unbalanced operation condition of a power grid are eliminated. A hierarchical clustering algorithm is used, the polarity and the amplitude characteristics of the transient state capacitive zero sequence currents of the initial ends of all feed lines are integrated to carry out classification on the feed lines, and a single feed line with the change trend of the transient state capacitive zero sequence current easier than the transient state capacitive zero sequence currents of other feed lines is selected as a fault line. The method improves the reliability of the line selection, lowers signal sampling frequency needs, avoids multi-layer signal decomposition, saves computing resources, and is easy to achieve.