Abstract: For the long-term progressive damage of the coal pillar dam body of the underground reservoir under the action of water immersion-periodic stress, taking a mine in Shaanxi Province as the engineering background of the underground reservoir project, the working face was analyzed through field investigation, theoretical analysis and laboratory experiments. The stress environment of the coal pillar dam body in the final mining stage, and the damage law of cyclic stress and water immersion to the coal body were explored. The results show that there are many secondary fissures affected by mining before the formation of the coal pillar dam, and after the formation is affected by water immersion and the mining of adjacent working faces, the stress is characterized by multiple cycles of loading and unloading; the compressive strength is 18.40 MPa, the breaking load is 36.11 kN, and the failure loads of 1, 2, and 3 cycles of loading and unloading are 35.27, 29.15, and 27.45 kN, respectively, indicating that the cyclic stress has the effect of aggravating the coal damage; with the increase of the number of loading and unloading cycles, the coal strength gradually decreases. The coal strength is 17.97 MPa under the condition of one loading and unloading without water immersion, and 10.78 MPa when loading and unloading three times in 21 days.