The relationship between the transmission line length and the operating states of transmission line， including the conductor temperature， equivalent icing thickness and wind speed， is analyzed based on the equation of state in transmission line. The corresponding method to calculate the transmission line length is also proposed by considering the operating states of transmission line. Correctness of this method is verified by the conductor tension and conductor temperature practical running data from Northeast China. Finally， the conversion equation is established according to relationship between the transmission line length and the operating states of transmission line， and then the actual distance to fault position is converted into the horizontal distance.