This study investigates the relationship between pile deformation and the horizontal resistance coefficient (m-value) of soil under lateral loading through horizontal static load tests on prestressed high-strength concrete (PHC) piles. Finite element simulations were conducted using m-values recommended by different design codes, and the numerical results were compared with experimental measurements. An iterative numerical method based on the finite element method (FEM) was proposed to determine the optimal m-value. The results demonstrate that the iteratively solved m-value and corresponding displacement predictions exhibit the best agreement with experimental data under varying load conditions. In a practical engineering case study, two different initial m-value were used for iteration, and the resulting converged m-value differed by less than 5%, validating the robustness and feasibility of the proposed method. This approach provides a reliable reference for engineering applications.