According to the theories of thin film calculation and optimization, a new global optimization method for multilayer film thickness calculation is presented. Firstly, a new group of local optimums is found out through genetic and mutation operation. Then, the solutions are accepted at a certain probability and will be calculated respectively in Adapted Simulated Annealing (ASA) process as the initial solutions. At last, conjugate gradient algorithm is utilized to search accurately and quickly. This genetic simulated annealing algorithm can effectively enhance the robustness of the algorithm and reduce the limitation of the searching area. The experimental results show that the proposed algorithm can calculate the thickness of 3-layer and 4-layer thin film within the range of 10 nm~5 μm and less than 1% calculation error