Based on the structural characteristics of the four circuit transmission lines on the same tower and the lines with/without electrical connection on the same voltage level, this paper analyzes the influence of the mutual inductance between the lines on zero-sequence directional protection when single line longitudinal fault occurs on four circuit transmission lines on the same tower, and calculates the amplitude and phase angle characteristics of zero sequence voltage and current on each line by using the equivalent transformation of the network topology. The analysis indicates that when the single line longitudinal fault occurs on four circuits on the same tower, there is a risk of mal-operation of zero sequence directional protection for the line without electrical connection on both sides; for the line with electrical connection on both sides, the zero sequence direction will not mal trip. Finally through the PSCAD, the model of four circuits on the same tower is established and the single-phase line break is simulated. Simulation results verify the correctness of the theoretical analysis.