The formation mechanism of gradient-index coatings by reactive magnetron sputtering is discussed. A practical modeling of gradient-index coatings is propose d, the relationship between refractive index of coatings and pressure of reactiv e gas is established. Next, we discussed the changing rule of partial-pressure o f reactive gas with time requivcd for obtaining a desired refractive index of g radient coatings under specific deposition conditions. A linear coating is tak en as example to illustrate how to get the relationship between partial-pressure of reactive gas and time.