The utility model discloses a grounding fault wire selector for a transformer substation DC system. The grounding fault wire selector comprises a signal generating source (1), a primary coil (2), a secondary coil (3), an O-shaped magnetic ring (4) and a Hall sensing element (5), wherein the primary coil (2) and the secondary coil (3) are wound on the O-shaped magnetic ring (4), the Hall sensing element (5) is installed in the O-shaped magnetic ring (4), the signal generating source (1) is connected with the primary coil (2), and the overall forms the grounding fault wire selector. The grounding fault wire selector adopts the effective combination of the primary coil and the secondary coil, realizes the effective searching and analysis of the positioning of the DC grounding fault branch with the cooperation of the Hall sensing element, is not affected by the positive and negative ground capacitance, is not interfered by external signals, and has the advantages of comprehensive data and rapid measurement.