Abstract: Based on date of surface topography of rock joint measured by laser profilometer, by transferring the height distribution into the slope distribution and taking spectral analysis of slope distribution, the anisotropic behavior of surface topography of rock joint is discussed in this paper. The general statistical parameters can supply the average measurement of certain behavior of individual profiles, but can not catch the behavior of anisotropy. However, the power spectral density can characterize both the shape of a profile at the large scale and the fluctuation at the small scale, therefore it is superior to the general statistical parameters. In this paper, a parameter called index for the accumulated power spectral density is defined to characterize quantitatively the anisotropy of surface topography of rock joint.