Control method of power supply system

The present invention relates to a control method of a power supply system, which is used in an electric vehicle. The electric vehicle includes a main battery, a motor controller, a voltage reduction device, a constant battery, and a vehicular load. The main battery is communicatively connected to a control unit. The constant battery is electrically connected to the step-down device through a charging power switch, and the vehicular load is electrically connected to the step-down device and the constant battery through a load power switch. Wherein, when the constant battery power is sufficient, the vehicle start process can give priority to starting the vehicular load, in order to achieve the shortest time to start the vehicle and the front or rear lights to give priority to the warning function of the oncoming vehicles. When the constant battery power is not sufficient, the vehicle start process gives priority to turn on the charging power switch, so that the battery can be charged first and the power output