Abstract: Root extinction coefficient β ,is one of the most important parameters in determining the vertical distribution depth of tree species.In this study,the root vertical distribution characters of Robinia pseudoacacia ,one of the main tree species for afforestation in Chunhua County in Weibei Loess Plateau,was investigated and its root extinction coefficient on different sites was determined.And the results showed 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 indicated that the vertical root distribution depth of Robinia pseudoacacia on southern exposition was deeper than that on southren exposition.And the distribution depth of fine roots( Φ <1 mm)was deeper than that of thicker roots( Φ <3 mm),which was in favor of the uptake of water and nutrients from deeper layers,helped the trees to adapt to the arid environment,and promoted the growth of the upper parts of the tree.