A novel technique based on non-unit transient to detect single phase high-impedance fault（HIF） is presented. HIF of high-voltage transmission line is difficult to be detected， and it is easy to be affected by the normal operation such as the switching events. Analysis results show that when a single phase fault happens in a transmission line， wavelet transformation of the zero-sequence current and the fault phase current will exhibit special characteristic， which can be used to detect HIF. Moreover， HIF and the normal switching events can be discriminated by using the difference of the corresponding high frequency component. ATP simulation results show that the criterions can be used to detect HIF with high reliability.