Power supply switching device and method, and instrument

The invention discloses a power supply switching device, a power supply switching method, and an instrument.The power supply switching device comprises an external power supply circuit and a battery power supply circuit, wherein output interfaces of both the two power supply circuits are used for being connected with a load power interface. The power supply switching device also comprises a switch unit including a control end, an input end and an output end, wherein the switch unit is connected into the battery power supply circuit in series via the input end and the output end; the control end is connected with the external power supply circuit, and used for acquiring a first preset level when an external power supply is connected with the external power supply circuit to control disconnection between the input end and the output end, and acquiring a second preset level when the external power supply is not connected with the external power supply circuit to control connection between the input end and the o