The present application relates to the technical field of vehicle-mounted charging, and provides a working mode switching control method and device for a vehicle-mounted charger, and a vehicle-mounted charger. The working mode switching control method for a vehicle-mounted charger comprises the following steps: obtaining a working mode switching signal of a vehicle-mounted charger, wherein the working mode switching signal comprises a switching signal for switching, in different power modules of the vehicle-mounted charger, the voltage of a direct-current bus controlled by means of an original power module to the voltage of a direct-current bus controlled by a power module to be started; according to the working mode switching signal of the vehicle-mounted charger, firstly controlling the power module to be started to start by using a current source control mode and adjusting a current given value according to a working condition until the required power of a system is borne; and controlling a control mode of the power module to be started to be switched to a voltage source control mode, and causing same to control the voltage of the direct-current bus. In the present invention, when the vehicle-mounted charger performs working mode switching, power supply interruption of a low-voltage end is avoided while the voltage stability of the direct-current bus is ensured.