The mixed-voltage multi-circuit transmission lines are widely used at home and abroad because of its advantages of large transmission capacity and small land resources. The cross-voltage fault between two different voltage-level lines is a common fault form under the mixed-line transmission mode. The literature has studied the influence of the grounding condition of the fault on the distance protection. No literature about non-grounding cross-voltage short-circuit is found, and it is more difficult to analyze. In this paper, the cross-voltage phase-to-phase fault is taken as the object, and its influence on ground and phase-to-phase distance protection is analyzed separately. Based on the method of calculating the fault current of mixed-voltage lines on the same tower in the composite sequence network, this paper calculates the variation law of the measured impedance when the non-grounding cross-voltage short-circuit occurs, draws the conclusion that the amount is related to the position of the fault point and the phase angle of the special phase of the two systems, and points out the difference between them and its cause. The performance of the phase-to-phase distance protection of the non-complete fault phase is discussed, and the possibility of maloperation is pointed out. Finally, a model of 500 kV/220 kV four-circuit transmission line on the same tower is built in PSCAD to verify the proposed analysis. This work is supported by National Key Research and Development Program of China:Key Projects of Smart Grid Technology and Equipment (No. 2016YFB0900600).