The invention discloses a protecting method for short circuit fault of supply arms of a traction substation, which adopts the voltage difference of the supply arms and the current of a leading phase to form novel electric railway feeder impedance protection. The novel electric railway feeder impedance protection adopts improved quadrilateral characteristics, and a parallelogram which is based on an R-axis can play the role of protecting when short circuit fault occurs among supply arms and can ensure that any ground fault of the supply arms does not act on an activity area. The impedance protection provided by the invention only needs the voltage of the supply arms and the current of the leading phase and can be easily realized in already running or newly-built traction substation. The method is theoretically not affected by inrush current, load conditions, fault types, initial phrase angle when in fault, system impedance and transition resistance. The test result shows that the principle has great sensitivity, selectivity and rapidity.