Small-scale roughness on surface is equivalent to a multilayer structure with same thickness and gradient refractive index. By using characteristic matrixes, the reflectivity of P waves of the effective multilayer model with different incident angle is calculated; the refractive index of the absorbing medium is a complex number in the calculation of Fresnel formula; COMSOL Mutiphysics is used to model and simulat the small-scale roughness and absorption. The calculation results show that both the small-scale roughness and absorption of the dielectrics can make errors in measurement of the refractive index. In conclusion, in order to get the precision of 10－5 , the refractive index of rough dielectrics should be measured by critical-angle technique, while it is better to measure absorbing dielectrics by Brewster-angle technique.