An on-board charging device includes an AC connector, an AC to DC converter and a detection circuit. The AC connector is configured to be connected to an electric vehicle supply equipment (EVSE), so that a protective earth terminal of EVSE is electrically connected to a protective earth terminal of the on-board charging device. The AC to DC converter is electrically connected to the AC connector, and the AC to DC converter is configured to convert an AC voltage provided by the EVSE into a DC voltage. The AC to DC converter has a reference ground terminal. The detection circuit outputs a detection voltage based on the voltage difference between the protective earth terminal of the on-board charging device and the reference ground terminal of the AC to DC converter. The detection voltage reflects whether the protective earth terminal of the EVSE is abnormal or not.