Abstract: To address the engineering problems associated with the reinforcement of high-filled subgrades on steep cross slopes, the chair-shaped sheet-pile wall has been improved into a composite structure integrating the functions of both supporting and retaining. Its applicability and advantages are elaborated from the perspectives of structural system and functionality, load-bearing characteristics, and road performance. To facilitate engineering calculation and structural optimization, static equilibrium equations are established based on the Winkler foundation beam model and the initial parameter method, by considering the internal forces continuity, displacement compatibility at structural characteristic sections, and boundary conditions. Analytical solutions for internal forces and deformations of the structure are obtained by combining with the elimination method and the matrix method. Verification calculations through an example structure shows that the analytical solutions closely match the finite element simulation results from a two-dimensional frame model using Midas GTS NX, with an error of less than 2%. In addition, the results from three-dimensional solid finite element simulations using Abaqus also agree well with the analytical solutions, with the errors of bending moments at the mid-span of beam and along the pile shaft being about 5%. These results demonstrate that the analytical calculation model and its solutions are both reasonable and feasible. In the solid finite element simulation, the bending moment of the beam near beam-pile joints exhibits an error of about 20% compared with the analytical solution, primarily due to the abrupt change in the beam's section modulus at these joints, which leads to abnormal stress levels in adjacent elements. These findings propose a minimally invasive composite structure and its analytical solution for reinforcing high-fill subgrades on steep cross slopes, and also provide a practical calculation method for similar subgrade reinforcement structures considering soil–structure interaction.