The invention discloses a spare power automatic switching method and device, wherein the method comprises the following steps that current input signals and voltage input signals which are obtained through proportionally converting collected current signals and voltage signals of an on-site switch and can be recognized by a microprocessor assembly are obtained; switch operation parameters which are obtained through converting collected operation parameters of the in-site switch and can be recognized by the microprocessor assembly are obtained; switch control information is generated according to the current input signals, the voltage input signals and the switch operation parameters; the on-off state change of the in-site switch is controlled according to the switch control information, wherein the on-off state change of the in-site switch is used for switching a spare power supply to the work state when a main power consumption power supply fails, and is also used for switching the main power consumption power supply to the work state when the main power consumption power supply recovers the normal state. The method and the device have the advantages that the problem of spare power automatic switching inaccuracy is solved, and the effect of accurate, flickering-free and fast spare power automatic switching is realized, and the realized functions are complete.