Abstract: In order to reduce the risk for development and using of large aperture telescope, structure function was discussed in this paper. Firstly, the relationship between structure function and Strehl ratio was gotten in theory, and the method for calculating structure function was founded. Then, after low order ones of Zernike polynomial were analyzed, simulating the low order disturbance, the Kolmogorov spectrum was used to simulate the high ones, getting the tendency of strehl ratio in different dimension. Because of the direct relationship between coherent length and structure function, and the influence of the coherent length on the performance of the large aperture optical system, a method for evaluating the coherent length was proposed based on maximum likelihood method.