The underground reservoir technology system is a technical method to improve the utilization rate of water resources in the western region and alleviate the water shortage in mining areas. A reasonable coal pillar width of the underground reservoir is an important guarantee for the normal operation of the underground reservoir. Taking the underground reservoir of Daliuta Coal Mine as the engineering background, this paper theoretically calculates the reasonable reserved width of the coal pillar dam body of the reservoir, puts for-ward the main influencing factors of the stability of the coal pillar dam body from the perspective of the combined action of mining stress and initial crustal stress, and analyzes the influence of different influ-encing factors on the stability of the coal pillar dam body. The research results indicate that: (1) the rea-sonable width of the coal pillar dam body in the underground reservoir of Daliuta Mine is 27.01 m; (2) The higher the mining height of the working face, the smaller the width of the coal pillar, the higher the level of deviatoric stress inside the coal pillar, and the lower the stability of the coal pillar; (3) The smaller the lateral pressure coefficient of the initial crustal stress field, the larger the range of crustal stress angle deviation from 0 °, the higher the deviant stress level in the coal pillar, and the lower the stability of the coal pillar.