The invention relates to a vehicle power supply system and method. The system comprises a storage battery and vehicle electric equipment. The system further comprises an ignition lock, an engine controller, a generator, a generator manager, a generator high speed control device and a generator low speed control device. The generator high speed control device and the generator low speed control device are located between a generator follow-up belt wheel moving gear and a generator rotating gear. After the gear ON of the ignition lock is switched on, the generator manager detects whether the engine rotating speed is larger than a set rotating speed or not in real time, and if yes, the generator low speed control device is controlled to be engaged with the generator follow-up belt wheel moving gear and the generator rotating gear; and otherwise the generator high speed control device is controlled to be engaged with the generator follow-up belt wheel moving gear and the generator rotating gear. By means of the vehicle power supply system and method, the problem of vehicle idling insufficient power is solved.