The invention provides an earth fault diagnostic device. The earth fault diagnostic device comprises a detector, an indicator and a charging capacitor, wherein one end of the detector is grounded; the other end of the detector is connected with one end of the indicator; the other end of the indicator is connected with a cathode of a main circuit; the detector is used for detecting a grounding electric signal in the main circuit; the indicator and the charging capacitor are arranged in parallel connection; the indicator is used for indicating whether an earth fault exists in the main circuit; and the charging capacitor is used for absorbing the grounding electric signal when the grounding electric signal exists in the main circuit. The earth fault diagnostic device solves the problem that in the prior art, when the performance of a current sensor is reduced or damaged and no earth fault exists, the current sensor reports an earth fault so that a control system breaks disconnects the main circuit of a locomotive and normal operation of the alternating-current electric locomotive is influenced, and can restart a vehicle to guarantee normal operation of the locomotive when an earth fault is reported falsely.