Abstract: Sitt mudstone, a widely distributed soft rock, presents shear strength properties that are critical for slope stability and geotechnical safety. However, the shear behavior of sitt mudstone under saturated conditions remains insufficiently characterized, despite the fundamental role of moisture in controlling rock strength and deformation. In this study, the purple-red sitt mudstone from the Hongshiya Formation in the Yezhutang area of Yunnan Province, China, was selected as a representative material. Laboratory direct shear tests were conducted under both natural and saturated conditions. The shear strength parameters of the sitt mudstone were obtained, and the degradation mechanism of moisture on its strength was analyzed. The roughness coefficient of the shear failure surface was obtained using 3D laser scanning technology, and then was used to quantitatively describe the damage characteristics of the shear failure surface of sitt mudstone samples. Results show that the failure modes of sitt mudstone samples are mainly shear failure and local shear failure. Under the local shear failure mode, the shear strength of saturated sitt mudstone significantly weakens, with a 50% decrease in cohesion and a 5% decrease in internal friction angle. That is, the weakening of cohesion by water is more pronounced compared to that of friction angle. The roughness coefficient of the shear failure surface of sitt mudstone in the saturated condition is smaller than that in the natural condition, and the wear degree is more significant. The results provide valuable insights into the shear strength assessment and moisture-induced damage characteristics of sitt mudstone in the Yezhutang area of Yunnan Province.