In order to get a better understanding of etching mechanism and provide optimization guidance for manufacturing process, a three-dimensional (3D) profile evolution simulator based on cellular model is developed to investigate the surface evolution of etching process, and discuss emphatically the effect of ions on the surface evolution. According to the solving problem for angle of ion incidence, a component fitting-based dimension reduction method is presented to convert a 3D surface fitting problem into a two-dimensional (2D) curve fitting problem, and achieve fast solution for the surface normal vector of the incident point. Compared with least squares polynomial fitting method, this method improves computational accuracy and efficiency of the ion incidence angle. The improvement on the accuracy of fitting is achieved by improving the selection method of surface cellular for fitting. The fitting method is applied to 3D simulation of silicon etching process, and the simulation results verify the simulated surface by comparing with relevant experimental results.