This paper introduces the measurement principle of a super-smooth surface by a Grazing X-ray Scattering(GXRS) method and an experimental facility based on an improved X-ray diffractometer. By using three kinds of wafers with different roughnesses as samples,the scattering distribution of the samples are treated by first-order vector perturbation theory. The results indicate that the calculated Power Spectral Density (PSD) by GXRS is in a good agreement with the results obtained from Atomic Force Microscope (AFM). It also analyzes the effects of the slit width of a detector and the divergence of incidence X-ray on experimental results,results show that when the slit width is 0.02 mm and the incidence divergence is 43″,the errors are both lower than 2% in the range of spatial frequency higher than 0.03 μm-1. It is concluded that the measuring errors decrease quickly with the reduces of the slit width and incidence divergence, and the PSD error also decreases quickly with the increase of spatial frequency. The repeatability of the experimental facility is better than 2.6%.