The utility model discloses an auxiliary power supply of an indoor transformer. The device comprises a mounting plate, a protective shell is fixed to the surface of one side of the mounting plate, a power supply placing plate is fixed to the surface, close to the protective shell, of the mounting plate, an auxiliary power supply is mounted on the power supply placing plate, movable sliding groovesare formed in the two sides of the surface, away from the protective shell, of the mounting plate, and a fixing plate is arranged on the surface, close to the movable sliding grooves; according to the utility model, the original direct fixing mode of the auxiliary power supply is changed into the mode of fixing the fixed power supply on the mounting plate; the device is fixed in an indoor transformer through a mounting plate. When the power supply needs to be installed, maintained or replaced; when the auxiliary power supply needs to be mounted, the electromagnet switch on the mounting plateonly needs to be turned off without magnetism, so that the mounting plate can be directly taken out, the auxiliary power supply can be conveniently taken out, the electromagnet is mounted on the fixing plate, the fixing plate can slide on the mounting plate, limitation of a mounting area is avoided, and a user can adjust the mounting position of the power supply.