The invention discloses a missort investigation device and an investigation method before installation of a single-phase resident ammeter. The investigation device is mainly constituted by an external power supply module and a photoelectric responding module, and is characterized in that the external power supply module is constituted by a lithium battery pack and a double-pole switch; the lithium battery pack comprises a charging circuit formed by sequentially connecting an input charging control circuit, a battery protection circuit, and a lithium battery, and a discharging circuit formed sequentially connecting an output boosting control circuit, a battery protection circuit, and a lithium battery; the double-pole switch is provided with a left switching port group and a right switching port group; one switching port group is connected with the input charging control circuit, and the other switching port group is connected with the output boosting control circuit. The investigation method is characterized in that the double-pole switch of the external power supply module is switched to the switching port connected with the output boosting control circuit, and is connected with a power supply wire outlet terminal of an ammeter, and then the photoelectric responding module is connected with the load connecting port of the ammeter, and the air switch can be connected to form a closed-loop circuit.