Graded index coatings are also called inhomogeneous coatings. Based on Drude theory, the relationship of dielectric constants between inhomogeneous coatings and coating materials was analyzed. The mechanism of preparing inhomogeneous coatings by multi-source co-evaporation was introduced. Subsequently, we discussed how to get the distribution of deposition rate v(x) of coating materials from the refractive index distribution n(x) of inhomogeneous coatings on the assumptions that the volumes of different materials were equal or not and the total deposition rate was constant or not, and some deposition rate expressions of different refractive index distribution such as linearity, sine, exponent, and hyperbola were given too. In the end, a simple example was given, in which the total deposition rate was constant and the change law of refractive index was linearity.