Abstract: Siltstone, as a common soft rock, has shear strength, which is an important parameter affecting slope stability and geotechnical engineering safety. Previous studies have focused less on the strength characteristics of siltstone in the saturated state, while understanding the effect of moisture on rock strength is crucial for guiding engineering construction. Therefore, using the purple-red siltstone from the Hongshiya Formation in the Yezhutang area of Yunnan Province as the research object, indoor direct shear tests were conducted under both natural and saturated conditions. The shear strength parameters of the siltstone are obtained, and the degradation mechanism of moisture on its strength is studied. The roughness coefficient of the shear failure surface is obtained using 3D laser scanning technology. The roughness coefficient was used to quantitatively describe the damage characteristics of the shear failure surface of siltstone samples. Results show that, the failure modes of siltstone samples are mainly shear failure and local shear failure. Under the local shear failure mode, the shear strength of saturated siltstone 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 siltstone in the saturated state is smaller than that in the natural state, and the wear degree is more significant. The results provide valuable reference for the shear strength assessment and moisture-induced damage characteristics of siltstone in the Yezhutang area of Yunnan Province.