When trains pass through the traction substation without putting down their pantograph， an arc fault between the different phases can be easily caused. In this situation， common relay protection will refuse to trip. The models of high arc-resistant fault and traction distribution networks are built and the characteristics of fault signals are analyzed. According to the characteristic of large amount of the third harmonic existed in the voltages of between phases， a new approach of relay protection for phase-to-phase faults is presented， which works by detecting the voltage value between faulted phases and the percentage of the third harmonic. This approach is not affected by the transient voltages of transformer's unload switching-on or the train pantograph switching-on， and can distinguish the fault between different phases from common faults correctly. It also possesses the advantages of simple and easily achieved.