The impact on operation of backup protection in medium voltage side caused by phase to phase short circuit fault in low voltage 10 kV bus side of 110 kV transformer is analyzed. Then， corresponding solutions such as fitting backup protection time， changing operation mode of system， etc， are proposed for reducing influence of low voltage side phase to phase fault on backup protection in medium voltage side. Through failure model simulation and theoretical calculation， the feasibility of previous solutions is verified， which may offer a reference for the setting calculation of power system.