Abstract: Coal mine underground reservoir water storage technology has become one of the important ways for the coordinated development of coal resource mining and water resource protection in western mining areas of China, and the stability of coal pillar dam body is one of the key factors for coal mine underground reservoir to be able to be used safely and for a long time. To investigate the destabilization characteristics of coal pillar dams in underground coal mine reservoirs under complex conditions, this paper is based on FLAC^3D numerical simulation and secondary development to investigate the influence on the stress evolution characteristics and stability of coal pillar dams under the sequential action of adjacent workings mining and water flooding in the mining area in a mine in northern Shaanxi Province. The results show that the stress distribution of the coal pillar dam body shows a bimodal shape after the mining of the working face is completed. The peak value gradually decreases with the increase of the width of the coal pillar. At the same time, the elastic core area accounts for less than 40% of the critical instability index when the width of the coal pillar is 20 m, and instability will occur. After water storage in the mining area, the increment of the plastic zone of the coal pillar dam body under flooding shows the change law of “stable-increasing-decreasing-stable”, which can well simulate the progressive damage characteristics of “seepage-weakening-damage-seepage” of the coal pillar dam body under flooding. The stress distribution of the coal pillar dam body gradually evolves into an arch-shaped distribution. It tends to average out, and the peak stress is greatly reduced, while the internal plastic zone is further expanded and the percentage of its elastic core zone is reduced. The smaller the width of the coal pillar is, the greater the reduction is.