The strength of the surrounding rock of the underground chamber is weakened under the condition of rich water, which threatens the excavation and operation of the chamber. In order to investigate the mechanical properties and microstructure of surrounding rock under water saturation, triaxial compression and SEM tests under different confining pressures and water saturation conditions were carried out. The results show that: (1) with the increase of saturation, the mechanical properties of sandstone are decreasing continuously, and the attenuation amplitude tends to be gentle when the saturated days reach 30 days. (2) the deterioration accumulation rule of the saturation deterioration of sandstone is relatively similar, the peak stress, cohesion, internal friction angle, elastic modulus and deformation modulus is similar, the saturated days accumulate faster before 15 days, and the accumulation rate is slowed after 15 days. (3) the fracture surface of sandstone is mainly composed of shell grain, mainly ductile failure, and confining pressure can inhibit the development of microcracks. The research results provide some reference for the study of the degradation law of rock mechanical properties under the action of water saturation.