Abstract: Solving Poisson model parameters in settlement analysis will fail when the iterative matrix is ill-conditioned. Based on the matrix transformation theory, the classical Landweber iteration formula is derived, and a polynomial Landweber iterative method of Poisson model for soft soil subgrade settlement is proposed. The formula is also given. According to the nonlinear least-squares principle, the methods and formulas for parameter factors in the formula are given. The observed data of soft soil roadbed in expressways are analyzed using this method. The results show that the polynomial Landweber iterative method can effectively avoid the convergence situation which the ill-posed iterative matrix. Ideal embankment settlement results are achieved when the parameter factors are accuately selected.