The degradation of sandstone,including mass degradation and variation of shear strength parameters,under the action of cyclic drying and water adsorption in high temperature season was simulated in laboratory. Sandstone samples were taken from Fanjiaping landslide in the Three Gorges Reservoir. Results indicate that 1 ) mass degradation rate and water adsorption rate fluctuated non-linearly under drying and wetting cycles,reflecting the dynamic development of fractures in the sandstone; 2) cohesion c and internal friction angle (p of sandstone reduced by a large margin in the initial stage of drying-wetting cycles, and then reduced steadily in the later stage,which is similar with the mass degradation law; 3) a method of determining the strength of sandstone in quantitative sense according to the extension angle of direct shear failure cracks was proposed. The research results are of practical significance to predicting rock deterioration in central China with humid climate and large temperature difference between day and night.