According to the self-similar nature of the optical surface in microstructure, its structure feature is described with fractal model independent of the scale. Fractal dimension of the polished surface is calculated by structure function. The influences of roughness RMS, error wavelength, measure scale, sampling length and the number of sampling points on fractal dimension are analyzed. On this basis, a new method which applies the first-order autoregressive fractal model to simulating the optical surface is presented. The influences of the definition scale and model parameter on fractal feature and fractal dimension are analyzed. The research indicates that the evaluation method is simple and independent of the scale in certain range when the microstructure of the optical surface is described by fractal dimension.