The calculation of the dispersion curves of pavements is studied in this paper. The dispersion curves of pavements used to be figured out by such means as the stiffness matrix approach or the finite element method. For pavement systems, the search for the roots of the frequency equation was beset with difficulties. The roots were mostly complex numbers and numerical overflow problems occurred frequently. The authors propose adding a high velocity layer under the sub-grade so as to convert the roots into real numbers. Using the bisection method to solve the modified model, the authors have found that the results coincide with those of the complex domain.