Disclosed in the present invention are a fast bus transfer method and device to reduce, by transferring between a main power supply and a backup power supply, the impact on a load connected to a bus caused by a power cut. The method comprises: 1) 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; 2) transferring the load on the bus to said backup power supply only when the amplitude is less than its limit value and the phase angle difference is less than 90°. The device comprises: 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 he/she can easily achieve fast transfer simply by setting the limit of the voltage difference at the moment when a circuit breaker of the backup power supply is closed.