The traditional method of identifying non-fault line tripping is based on the detection of only the sudden variation of line power flow and is likely to cause unintended operation of stability control devices under such conditions as light loading after a system swing. This paper presents an analysis of the measured impedance characteristics at one end of the transmission line under different conditions， reveals the mechanism of the measured impedance having distinct different values for non-fault line tripping out and power swing， and then proposes a new method of identifying non-fault line tripping based on local impedance measurement. Digital simulation results are given to validate the new method.