To accommodate the alignment of approach roads on both sides, some long-span cable-stayed bridges with truss-deck composite girders adopt an asymmetric cross-sectional arrangement of traffic lanes. This eccentric distribution of dead load induces significant torsional effects that conventional cable-force-optimization methods fail to address adequately. To overcome this limitation, this study proposes a novel stepwise cable force optimization framework to mitigate the adverse impacts of dead-load eccentricity. The proposed strategy exploits the decoupled characteristics of vertical bending and lateral bending-torsion in left-right symmetric cross-sections, sequentially performing average cable force optimization followed by differential cable force optimization. Specifically, this work investigates the equivalence principle that allows the plate–truss composite girder to be represented by a single-beam model for efficient analysis. Then, a B-spline curve is employed to approximate the dense cable force distribution using a limited number of control points, and a gradient-based algorithm is utilized to minimize the structural strain energy. The results demonstrate that the proposed method effectively reduces the torsional moment and deformation angle induced by eccentric dead loads, leading to a more rational internal force distribution and desired configuration. Moreover, the stepwise optimization strategy significantly reduces computational cost through problem decoupling and dimensionality reduction, while maintaining optimization accuracy and convergence stability.