A smooth switching device and method for double power supplies. The devices includes a main/backup power supply switching unit (1), a relay control unit (3), a charge management unit (2), a main power supply switching auxiliary unit (4) and a backup power supply switching auxiliary unit (5). The device and the method enable the main power supply (200) and the backup power supply (300) which supply power for electric equipment (100) to be switched smoothly, so that it is not easy for the electric equipment (100) to be disconnected with the power supplies and the fluctuation of a load voltage is small. Moreover, the device doesn't need a bulk capacitor used for auxiliary switching and doesn't need to be used together with a radiator.