Abstract: There is a distinction between the film optical characteristic calculated in theory and the optical characteristic appearing in actual measurement. It can be caused by many reasons, such as the incomplete theoretical model, the structural change when the film is stored away for a long time and the fabrication error. The corresponding methods in allusion to the cause to eliminate or avert the error is proposed. The film thickness and refractive index is the key to film preparation.In this paper, we analyzed the error of film thickness and refractive index caused by the optical control system, and also analysed the allowable error. In the end, we put forward the thought of computer real time optimization.