A power distribution unit and a fault detecting method applied in the power distribution unit are disclosed herein. The power distribution unit includes an input terminal, an insulation fault detection circuit and a processing circuit. The input terminal is electrically coupled to a positive power line and a negative power line, and configured to receive a high voltage direct current (HVDC) voltage. The insulation fault detection circuit is configured to detect an insulation resistance value between a ground terminal and the positive power line or the negative power line. The processing circuit is configured to output a warning signal according to the insulation resistance value.