Abstract: Coal mine underground reservoir is one of the key technologies to solve the protection and utilization of water resources in western mining areas. The basis for maintaining the safe operation of the reservoir is engineering safety, and the dam body is the core to ensure engineering safety. This paper studies the safety and stability of coal pillar dam of underground reservoir in coal mine. The stress variation (vertical stress and horizontal stress), fracture development and expansion, displacement variation and other multi-parameter variation laws during the failure of coal pillar dam are comprehensively analyzed based on mechanical model, similarity simulation and numerical calculation methods, from the angle of roof rock pressure (vertical stress) and water storage height (horizontal stress). This paper studies the variation characteristics of various parameters during the failure of coal pillar dam, and puts forward the key information of coal pillar dam failure precursors, which provides the basis for the safe operation monitoring and early warning of on-site coal mine underground reservoir. The results show that under the action of vertical stress, the location of coal pillar dam damage caused by overburden stress is the dam roof on the side of underground reservoir of coal mine. The smaller the size of coal pillar is, the easier it is to produce stress concentration. The greater the bearing stress is, the greater the possibility of damage is. Based on this, the failure critical point of coal pillar dam is obtained. Under the action of horizontal stress, the coal pillar dam body experiences three processes: elastic stage, initial failure stage and failure increase stage. The coal pillar dam body shows obvious displacement only after reaching the ultimate strength. At this time, the coal pillar dam body is damaged. After the horizontal stress decreases, the displacement has a rebound trend, but the overall stability of the coal pillar dam body is affected and it is not suitable to continue water storage. The analysis of the main factors causing the instability of the coal pillar dam shows that under the combined action of vertical stress and horizontal stress, the instability of the coal pillar dam is mainly at the top plate, bottom plate and middle position of the small-size dam near the reservoir. Taking Shangwan mine as an example, the maximum bearing head height of the coal pillar dam is 50m, but the coal pillar dam has been damaged at this time. Even if the water level is lowered, the overall stability of the coal pillar dam is irreparably damaged. Comprehensive study of the characteristics and laws of coal pillar dam instability in coal mine underground reservoirs, combined with displacement changes and the physical and mechanical properties of coal pillar dams, obtained the precursor information of coal pillar dam failure, that is, respond to the reservoir before reaching the ultimate strength under the action of external force. For safety measures, it is recommended to focus on monitoring the stress change information of the coal pillar roof with a smaller size on the reservoir side during on-site monitoring and early warning to ensure the safe operation of coal mine underground reservoirs. The research results provide a basis for monitoring and early warning methods for the safety and stability of coal pillar dams in coal mine underground reservoirs.