When the impedance relay with offset characteristic is applied to the backup protection for transformer phase-to-phase faults，its reliability and sensitivity always cannot meet the requirement.A negative sequence impedance relay（NSIR）is presented.Its measuring impedance is not dependent on phase shift of the Y，d connection of the transformer and it can lock effectively while line broken happened to potential transformer or current transformer.Simulink model used to verify performance of NSIR is built.Simulation results show that NSIR can act reliably when various phase-to-phase short circuit faults take place，and it also proves with an engineering practical example that NSIR is a sensitive and reliable backup protection for transformer phase-to-phase faults.