Soil drilling method is applied for root distribution characters investigation of the main forestation tree species on the loess plateau. The results indicate that there is obvious difference in the root distribution characters on different sites, both root density and root biomass on southern slope are smaller than that on northern slope. There are no obvious differences in root distribution depth on stand of different age, but root biomass and root density increased with age. Root extinction coefficient β, is one of the most important parameters in determining the vertical distribution depth of tree species. In this study, results showe that the value of β on northern exposition was more than 0.982, while the value of β on southern exposition was less than 0.982, which indicates that the vertical root distribution depth of Robinia pseudoacaciaon southern exposition is deeper than that on southren exposition. And the distribution depth of fine roots (Φ<1mm) is deeper than that of thicker roots(Φ<3mm), which is in favor of the uptake of water and nutrients from deeper layers, helps the trees to adapt the arid environment, and promoted the growth of the upper parts of the tree.