The optical transmission function is modified by taking account of the influence of thickness variation Δd on transmission of thin films. Meanwhile, we introduce an optimization technique and define an evaluating function, such that a simple method is developed based on the envelope curves of the optical transmission spectrum over the wavelength range 500—2700nm at normal incidence, which is used for the determination of the equivalent refractive index n, equivalent extinction coefficient k, average equivalent thickness d and thickness variation Δd of the ordered Ag/AAO nano-array composite structure. The relative error is about 0.3% in the calculation of average equivalent thickness . The simulated transmission of Ag/AAO nano-array composite structure is in satisfactory agreement with the measuremental values. All these results show that with this simple method the optical parameters of Ag/AAO nano-array composite structure can be calculated of accurately and the results are consistent with the experiment.