Abstract: A method is proposed to determinate the reflective,extinction coefficient and thickness of thin film material at the same time by means of transmission spectral and simulated annealing algorithm.The strict calculating formula and program diagram are provided.To check the accuracy and feasibility of the method,numerical simulation is first done,and then the SiNx thin films are tested.Simulated results are highly closed to theoretical values,and reproduced curve in terms of the experimental results conforms to initial experimental curve.This method has a lot of advantages such as no damage,simplicity of measurement,facility of manipulation,good stability,fast convergence and high accuracy.