Based on distributed parameter model of transmission lines, this paper proposes a new fault locating practical algorithm suitable for the four-circuit transmission line on the same tower differential protection device. Firstly, the transmission line parameters are carried out phase-mode transformation, independent double-ended with the positive sequence fault components are selected, and according to the theory of establishing a dual-port transmission line fault location equation, the accurate fault location is obtained. The method is simple and has less calculation amount, which avoids the complex process of a general ranging method, such as calculating the hyperbolic functions, transcendental equations, Newton iterative method and search methods. PSCAD/EMTDC simulation results show that the algorithm has high fault accuracy, and is immune to fault distance, transition resistance, fault type and power angle difference of double sides power sources, so it has practical value of engineering. This work is supported by National Natural Science Foundation of China (No. 51177066).