When a fault occurs in mixed-voltage four-circuit transmission lines on the same tower, because of their close physical proximity, there is mutual inductance between phases, but also there is the fact that the existence of mutual inductance between different voltage systems will have a great influence on fault analysis and protection. The basis for establishing a zero-sequence decoupling circuit is explained for the circumstance that a single-circuit fault occurs in a mixed-voltage four-circuit transmission line on the same tower under weak-electric and strong-magnetic conditions and with a strong-electric connection. Then, the performance of the zero-sequence direction relay and the main influencing factors are analyzed. The results show that the zero-sequence mutual inductance between different voltage level systems has a negligible effect on the performance of the zero-sequence directional relay under weak-electric and strong-magnetic conditions. The zero-sequence mutual inductance, electrical strength and fault location comprehensively affect the performance of zero-sequence directional relay under strong-electric connection conditions. An ATP-EMTP 1 000 kV/500 kV model of the mixed-voltage four-circuit transmission line on the same tower is established and the performance of the zero-sequence direction relay under single-circuit fault conditions is simulated to verify the accuracy of the theoretical analysis. The work is supported by the Scientific Research Project of China Electric Power Research Institute (No. SGDK0000JBJS1907459).