Aiming at the problem of standby power auto switch device not meeting the demandabout three incoming power supply in the process of installation at 110 kV Yanchi Substation,byimproving software logic,transforms the PSP641U standby power auto switch device of State PowerNanjing Automation Company into 110 kV special standby power auto switch device. Theapplication result shows that the improved software logic satisfying the operation need of 110 kVsubstation under different operation mode,can accurately put into standby power source when thebus lose voltage,effectively improves the reliability of power supply for Yanchi Substation.