Abstract: A fast bus transfer method and a device reduce the impact on a load connected to a bus caused by a power cut. The method includes calculating the amplitude of the voltage vector difference between the bus and the backup power supply and the phase angle difference between the bus and the backup power supply; and transferring the load on the bus to the backup power supply only when the amplitude is less than its limit value and the phase angle difference is less than 90°. The device contains a detection module, a calculation module, a comparison module, and a transfer module. By calculating the real-time residual voltage, it is unnecessary for a user to know all the details of the residual voltage characteristic, and can achieve fast transfer simply by setting the limit of the voltage difference when a circuit breaker of the backup power supply is closed.