When a cross-voltage fault occurs in the four-circuit lines of the same tower, the existence of zero-sequence mutual inductance will have a great influence on the fault current. When the cross-voltage unground-fault occurs in the mixed-voltage four-circuit lines of the same tower, the characteristics of current of the fault phase and the performance of the phase selector are analyzed, using the method of an improved composite sequence network to calculate the fault current. This paper uses a phasor diagram to examine the characteristics of the cross-voltage fault current and its effect on the phase selector based on the sudden-change of phase-to-phase current. Looking at the possible mis-selection of phase selector based on the sudden-change of phase-to-phase current, a comprehensive phase selection method based on the combination of sudden change of phase-to-phase current and sudden change of phase-current is proposed to achieve a correct selection. Based on PSCAD/EMTDC, a large number of simulation experiments are carried out in the mixed-voltage four-circuit lines of the same tower. The simulation results verify the accuracy of the theoretical analysis. This work is supported by Key Project of Smart Grid Technology and Equipment of National Key Research and Development Plan of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. 52094017000W).