Abstract: The shear strength characteristics of loess joints significantly affect the stability of project with joint loess. Based on a large number of field investigations,the suitable sites are selected to collect the loess vertical joints. The first is to classify different joint surface morphologies for sample preparation. Then,laboratory direct shear test is used to study the influence of water on loess joint shear strength parameters,and comparing with lab large-scale direct shear test to study the influence of the size effect on the shear strength. Finally,according to the results,the law of joint shear stress and displacement is studied. The results show that the loess joint shear stress-shear displacement relationship curve belongs to nonlinear yield shear type curve;loess joint shear strength is corresponded with Mohr Coulomb criteri- on,and cohesion is so small that almost negligible;as the water content increases,loess joint peak friction angle initial tends to a lesser extent,when more than a moisture content value,the peak friction angle is reduced significantly. The loess joint strength with the variation of the amount of water available index form are described;the sample size mainly affects the joint surface’s contribution to the shear strength,and the greater the size is,the greater shear strength is; the yield shear stress and displacement curve is nonlinear shear curve,and the constitutive model is built to study the initial shear stiffness. The results show that the initial shear stiffness decreases with the increase of water content,and increases linearly with the increase of normal stress.