Abstract: With the continuous development and expansion of China's power grid, there has been an increasing shortage in the transmission channel resources, thus the multi-circuit transmission lines on the same towers are gradually being widely used. The EHV / UHV long distance transmission lines generally use the differential protection as the dual main protection, which forms the dual configurations of the main protection with the longitudinal distance protection or the longitudinal direction protection. However, these protection configurations are not completely applicable to the double-circuit transmission lines because the sensitivity of the differential protection is affected by the load current and the distributed capacitive current. The judgment of the longitudinal distance protection and the longitudinal direction protection is affected by the mutual inductance between the lines. This paper proposes a protection approach for double-circuit transmission lines on the same tower based on the Tellegen's quasi-power theorem. The approach is not affected by the electromagnetic coupling, the distributed capacitance and the load condition. This paper verifies the applicability of the time synchronization method in the double-circuit transmission lines. A simulation model for the ultra-high voltage (UHV) Double-circuit Transmission Lines on the Same Tower demonstration project in China is established by using the MATLAB/Simulink, and the extensive simulation studies guarantees the effectiveness of the proposed protection approach.