In this paper, based on the Drude theory and Lorentz-Lorenz theory and started with the dielectric constant,the refractive index of mixed coatings is discussed. Under some special conditions such as, two materials′ molecule sizes is similar, the total deposition rate is constant and their deposition rate change is linear, the refractive index expression is obtained. Then the optical characteristics of gradient refractive index coatings are discussed from different aspects such as negative change or positive change, single layer or multi-periods, different periods or different single layer′s thickness. At last, the preparation, applications and some problems of gradient refractive index coatings are discussed separately.