The complex electromagnetic coupling between polars of double-circuit HVDC transmission line increases the difficulty of fault line selection. According to the decoupling theory of four-parallel transmission lines on the same tower, the voltage variation of double-circuit HVDC transmission line is transformed into four separate voltage components: a ground mode component and three line mode components. Further, the transformation is employed to analyze the characteristics of ground mode component and line mode components of voltage variation under different kinds of fault. Thereby a method for fault line selection of double-circuit HVDC transmission line is proposed based on the amplitude and polarity of ground mode component and line mode components of voltage variation. Finally, the simulation results of a double-circuit HVDC system model based on PSCAD/EMTDC show that the proposed method is effective and immune to the fault resistance.