Abstract: To address the engineering challenges associated with high-filled subgrades on steep cross slopes, including insufficient global stability, differential settlement and significant construction disturbance, a novel composite structure is proposed through the optimization of the chair-shaped pile-slab wall system.The proposed structure enables coordinated vertical support and lateral retaining actions, thereby improving the mechanical performance and serviceability of reinforced subgrades relative to existing subgrade retaining structures.To facilitate engineering calculation and structural optimization, an analytical solution is developed based on the Winkler foundation beam model and the initial parameter method. The static equilibrium equations of the structure are established firstly through incorporating internal force continuity, displacement compatibility and boundary constrain. Analytical expressions for internal forces and deformations are subsequently derived by combining the elimination method and the matrix method. An engineering case is employed to validate the proposed analytical method through comparisons with numerical simulations of finite-element models. The internal forces and deformations obtained from two-dimensional numerical simulations applying Midas GTS NX shows discrepancies of less than 2% relative to the analytical solutions. After excluding local stress concentration areas induced by the abrupt change in stiffness near the beam-pile joint, the three-dimensional simulations applying Abaqus also exhibit well with the analytical results, deviations of approximately 5% in the bending moments along the beam and pile shaft. The verification results demonstrate that the proposed theoretical calculation model and analytical apporch are reasonable and reliable, and sufficiently accurate for practical engineering applications. This study enriches the reinforcement structure system and calculation theory for steep-slope high-fill subgrades, but also provides a practical calculation method for the analysis of similar structures considering soil-structure interaction.