The invention relates to the technical field of power transmission and distribution, and discloses an accessory device and method for a secondary power supply system of a power transformer substation. According to the device employed in the method, a lightning-protection circuit is connected between the input end and grounding end of the device located between power supply equipment and load equipment. The lightning-protection circuit is connected between the input end and grounding end of the device. The device consists of a rectification circuit, a power adjustment circuit, an inversion circuit and a microprocessor. The rectification circuit is connected with the inversion circuit through the power adjustment circuit, and the power adjustment circuit is connected with the microprocessor. The control end of the microprocessor is connected with the inversion circuit. The data transmission end of the microprocessor is connected with a touch screen. The device enables a lightning residual voltage entering a load to be smaller, facilitates the protection of the load from being struck by lightning, improves the use safety, and prolongs the service life. Moreover, the device is strong in anti-interference capability. The device is reliable in operation, is simple in manufacture, and is high in safety.