Traditional measurement of optical inhomogeneity always obtain a globle inhomogeneity including surface parallelism, surface shape and systematic errors. Based on the Lorentz-Lorenz dispersion equation, the relationship between optical inhomogeneity and wavelength is analyzed. The difference of optical inhomogeneity in different wavelengths is much smaller than the required precision of optical components′ inhomogeneity in practical application, according to which and optical material′s dispersion properties, absolute measurement method of optical inhomogeneity with dual-wavelength phase-shifting interferometry is proposed. The measurement principle and the proposed formula are both confirmed in computer simulation and optical experiment.