Abstract: In order to understand the effect of forest roots on loess shear strength better,triaxial compression tests were carried out to determine the stress-strain and strength characteristics of forest root-loess composite.The shear strength of the composite was investigated in terms of various root diameter,root distribution,water content of composite and confining pressure.Results proved that the direct shear strength can be described by Coulomb Equation,i e τ_f=σtanφ+C.It was found that with the increase of water content from 12.72% to 15%,the shear strength of horizontal composite decreased,while the shear strength of vertical and complex root-loess composite increased.When the water content of composite in the steady state,shear strength of three kinds of root-loess composites increases with the increase of root diameter.And when the diameter of root system and water content of the composite are constant,the forest rootloess composite of complex root system has the largest shear strength,then the vertical root system,and the horizontal one is the least.