Abstract: In order to study the deterioration rules of shear strength in sandstones under wetting and drying cycles, sandstone specimens are tested after 1, 3, 6, 10 wetting and drying cycles in the environment of pH=7, 9, 4. Through UCS and TCS after different cycles, the cohesion and internal friction angle of sandstone are calculated, and the relation formulae between them and number of cycles N are determined, respectively, thus the shear strength is obtained under wetting and drying cycles in acid and alkali environment. The results show that the shear strength of sandstone decreases with the increase of cycles. In the early stage of cycles, the deterioration effect is comparatively significant, and the deterioration of shear strength is very large, then, the strength deterioration is relatively small. In the acidic environment, the deterioration of shear strength in sandstones is the most serious, slightly reduced in the alkaline environment, and the neutral condition is the lightest. The deterioration of shear strength in dried samples is much lower than that of wet samples.