Abstract: In order to master the mechanical strength characteristics of the colluvial deposit and the influence of water content on it, a typical loose colluvial deposit of landslide in southwest China was taken as a research object. A series of large-scale direct shear tests were carried out under different water content conditions and normal stress, the macro mechanical characteristics such as stress-strain, particle breakage, volumetric strain, shear plane failure and so on in the shear process of the colluvial deposit were analyzed, and the determination method suitable for its shear strength was proposed. The research indicates that the colluvial deposit shows obvious strain hardening characteristics under different water contents, and the shear stress displacement curve has no significant peak value, but there is a certain degree of stress "jump" phenomenon. Besides, the shear process produces a relatively obvious particle crushing phenomenon, and the relative particle crushing rate increases with the decrease of water content or the increase of normal stress. Meanwhile, the tests show that the shape of shear failure surface is not an ideal plane. With the increase of water content, the shear surface gradually changes from an irregular shape of "convex up and concave down" to a flat failure surface with a certain angle with the shear joint in space. Combined with the analysis of the characteristics of the normal stress, shear stress and stress ratio on the shear plane during the shear process, a method for determining the shear strength of the accumulation based on the stress ratio is proposed, and the effectiveness of this method is verified. This method is not only simple to operate, with little human interference, but also can better reflect the essence of the shear failure of the soil rock mixture. Finally, the tests reveal that water has an obvious softening effect on the colluvial deposit. The apparent cohesion c and internal friction angle φ linearly decrease approximately with the increase of water content, and the deterioration effect of apparent cohesion is greater than that of internal friction angle. The research results can provide a reference for the determination of shear mechanical properties and strength values of deposits in the southwest mountain area.