Abstract: Genetic algorithm based on elite selection strategy has good ability of global optimization and robustness. The non-linear least square method with Levenberg-Marquardt algorithm has fast and efficient local convergence features. The feasibility and effectiveness of reverse engineering of optical constants and thicknesses by the combination of both algorithms were studied. The results indicate that the combined fitting method can be used to calculate the optical constants and thicknesses of multilayer with great feasibility and efficiency under the actual restrictions, especially in finding the global optimum solution. At the same time, based on the theory of the whole spectrum fitting, the combined fitting method was applied to calculate the optical constants and thicknesses of 15-layer high-reflection multilayer and accurate thin films' parameters were obtained. The fitting experiments show that the combined fitting method is practical in reverse engineering of optical constants and thicknesses of multilayer, and it can be used in optical reverse engineering.