The invention relates to a vehicle-mounted charging system of an electric vehicle, comprising a vehicle-mounted battery, a single / three-phase power conversion module, a direct current charging device, a charging contactor, a signal detection unit and a micro control unit (MCU), wherein the single / three-phase power conversion module, the direct current charging device, the charging contactor, the signal detection unit and the MCU are respectively connected with the vehicle-mounted battery; the direct current charging device comprises a voltage boosting module and a voltage reduction module which can charge the vehicle-mounted battery through boosting or reducing the voltage of an external direct current charging power supply; and the single / three-phase power conversion module directly multiplexes a vehicle-mounted inverter of the electric vehicle with an alternating / direct current bidirectional inversion function. In the invention, three charging modes of the electric vehicle are integrated in one vehicle-mounted charging system and are flexible and selectable, the vehicle-mounted charging system can be charged by three different charging voltages, i.e. external direct current voltage, single-phase alternating current voltage and three-phase alternating current voltage, and four alternating current charging modes can meet the diversity requirement of the charging modes of current electric vehicles; and meanwhile, the original vehicle-mounted inverter components can be multiplexed, thereby effectively reducing the cost.