Abstract: The protection and utilization of groundwater resources in coal mining process is the prerequisite and neces- sary condition for realizing green mining at the ecological fragile mining area in western China. Among them,the exten- sive application of water-preserved mining and underground reservoir technology provides the possibility for solving the problem of coal-water co-production. In this paper,with the background that the stability of the coal and rock boundary of underground reservoir changed under the conditions of repeated immersion in water,the sandy mudstone of the No.3-1 coal seam roof of Wulanmulun Mine was investigated. The natural water absorption law of the sample was obtained by non-destructive water immersion test,and the water immersion time of the sample water content reaching saturation state was obtained. According to the immersion time,the sample was dried-saturated and repeatedly immersed in wa- ter. It was found that the water content increased exponentially with the number of immersion, whereas the shear strength and the number of water immersion in the sample decreased exponentially. The cohesive force decreased with the increase of the number of immersion times,while the internal friction angle increased first and then decreased,in- dicating that the uniformity of the material,and the shape of the fracture surface and the shape,size and direction of the fracture caused the internal friction angle to increase. The cycle immersion later was the dominant factor to make the internal friction angle decrease. Based on the relationship between cohesion,internal friction angle and number of dry-saturated cycles,the Mohr-Coulomb criterion was established based on the number of dry-saturated cycles. The a- coustic emission counting curve had a strong correspondence with the shear stress curve. The acoustic emission event increased significantly at the moment of sample failure,corresponding to the obvious rising phase of the cumulative a- coustic emission counting curve. The sample mainly had a low RA value during the failure process,and the failure time corresponded to a high RA value,i. e. ,the common type of crack in the failure process of the specimen was tensile crack,and the moment of failure corresponded to the obvious shear crack. These results show that the strength of coal and rock pillars in the groundwater reservoir boundary changed significantly under the action of water immersion-water loss during the impoundment-pumping process in the groundwater reservoir. It is suggested that the weakening effect of water on the strength of boundary coal and rock pillars should be considered when constructing the underground reser- voir.