Based on the terahertz time-domain reflection spectroscopy, the optical parameters of anhydrous D-glucose polycrystalline, i.e. the refractive index and the absorption coefficient were extracted by using iterative and genetic algorithm, respectively. After comparing and analyzing the two algorithms we had drawn the following conclusions: first, the calculation efficiency of iterative algorithm was improved using the solution of weak absorption approximation as initial values. However, the iterative algorithm was sensitive to the initial values. When the big difference between the initial values and real values existed, the accuracy of optical constants would be affected; Secondly, the genetic algorithm was not insensitive to the initial populations. It ensured the convergence of the algorithm and the population diversity through the design of parameter coding, initial population, genetic manipulation, parameter control and constraint condition. Last, compared with the iterative algorithm, the optical parameters obtained by the genetic algorithm had higher accuracy. Therefore, we suggest that the optical parameters of materials with higher accuracy based on the THz spectroscopy can be obtained by using an intelligent optimization algorithm.