The invention provides an electric vehicle precharging device, which comprises a precharging auxiliary device connected on positive and negative electrodes of a DC bus, wherein the precharging auxiliary device further comprises an electronic control unit and a flyback conversion circuit; the electronic control unit is used for receiving a DC bus voltage signal and a power battery voltage signal, and controlling the charging of a high-voltage DC bus capacitor by electric energy from a vehicle-mounted energy storage device in a precharging process; and the flyback conversion circuit is used for transmitting the electric energy from the vehicle-mounted energy storage device to the high-voltage DC bus capacitor for charging when precharging begins under the control of the electronic control unit. The electric vehicle precharging device and a precharging method thereof can significantly shorten the precharging time of high-voltage circuits of electric vehicles and hybrid-power vehicles and reduce an inrush current when a main relay is switched on.