Optical property analysis and preparation of graded index coatings can be approximated by subdividing it into a stack of many homogeneous layers. In this paper, layered medium theory is introduced firstly, which clarifies how to subdivide graded index coatings into homogeneous layers. Then a stratification merit method to get optimal layered number is put forward. At last, linear graded index coatings is taken as an example to illustrate how to obtain the optimal layer number of graded index coatings. Our discussion shows that the optimal layer number of graded index coatings depends on the coating thickness and changing speed of refractive index. When refractive index changes in given range, the layer number of graded index coatings decreases firstly and then increases with the increasing coating thickness.