A mathematical model that double-circuit line in the same tower is connected to a single-circuit line in series is established. The effect on the common distance protection is analyzed. The result indicates that the effect is relevant to system operation mode, line parameters and the fault location, and the sensitivity of distance protection should be verified when setting the settings. Sensitivity check software is compiled, which can provide the calculation and display of measurement impedance trajectory, and the sensitivity displayed as a graphical mode can be referred. Two solutions for the sensitivity of the distance protection are proposed. A RTDS simulation model based on the actual parameters on site and connected with real line distance relays is established. The confirmatory test based on the RTDS simulation indicates that overreached zone 2 and pilot distance relay maybe not operate if the sensitivity is incorrect. Finally, it is emphasized that the special connecting mode should be avoided when planning and designing primary system.