To address the issue that zero-sequence unbalanced currents caused by parameter asymmetry in non-transposition same-tower transmission lines seriously affect the reliability of zero-sequence differential protection, this paper proposes a method to quantitatively analyze the impact of asymmetry on differential protection and to evaluate protection adaptability. First, an impedance decoupling model is constructed using an improved six-sequence component method. Subsequently, the degree of asymmetry is introduced to calculate both zero-sequence longitudinal differential current and zero-sequence transverse differential current, and the influence mechanism of asymmetry on protection performance is theoretically analyzed. Finally, simulation results using PSCAD/EMTDC demonstrate that increased asymmetry leads to a rise in zero-sequence longitudinal differential current, forcing higher protection settings to prevent maloperation during external faults, thereby reducing the ability to withstand transient resistance. Furthermore, the zero-sequence transverse differential current not only increases with asymmetry but is also affected by fault location, resulting in poor adaptability in non-transposed lines. The research findings provide a reference and basis for the setting and improvement of differential protection in non-transposed transmission lines.