Ultra-smooth optical surfaces are very important in widely fields. They’re not only used in optics, but also in the electronics. Ultra-smooth surfaces are difficult to process, because the rms is less than 1nm. The process methods have Teflon Polishing, Float Polishing (FP), Magnetorheological Finishing (MRF) and Ion Beam Figuring (IBF) etc. Compared with conventional polishing, IBF have higher processing quality and efficiency. Low-energy (<2Kev) IBF can form the self-organized nanopatterns on optical surfaces. Since IBF is a non-contact method; there is no edge effect during the process. We can change the ion beam parameters to get dot or ripple pattern on substrate. Only the self-organized ripple pattern is discussed in the paper. For the prediction of process parameters, the principle theories Sigmund theory and BH model are used the interplay between the angle of ion beam incidence, ion flux, incident energy and substrate temperature leads to the self-assembly, which are considered by these theory. In this paper the angle of incidence and incident energy are mainly researched on. Processing nanopatterns on Si has been simulated by SRIM program with these theory and the results reveal several laws in the process. It is believed that these laws will help us to well predict the ion beam parameters and lead IBE experiments.