Abstract: The coal pillar dam is a key structure for ensuring the long-term safe operation of underground reservoirs in coal mines. However, existing studies do not sufficiently reveal the degradation mechanism of mechanical parameters of coal pillar dams under the coupling effect of mining disturbance and water immersion, as well as its influence on the distribution of pore pressure and the evolution of plastic damage in coal pillar dams. There is also a lack of quantitative research on the width of the water-blocking zone in the central part of coal pillar dams. To address these issues, FLAC3D was used to simulate the progressive degradation process of mechanical parameters of coal pillar dam under water immersion. On this basis, the variation patterns in vertical stress, plastic zone, pore pressure, and water-blocking zone of coal pillar dam under the action of mining disturbance and water immersion were analyzed. The results showed that water immersion caused the internal vertical stress distribution of the coal pillar dam to transform from a "saddle-shaped" pattern to a "single-peak" pattern, with the horizontal deformation increasing sharply and the proportion of the plastic zone increasing significantly. The water-blocking zone in the central part of the coal pillar dam exhibited a significantly inverted trapezoidal and non-uniform distribution, and the width of the water-blocking zone was greatly reduced. As the width of the coal pillar dam increased, the width of the central water-blocking zone increased, the degree of stress concentration decreased, the stress distribution on both sides became more uniform, and the effective load-bearing zone expanded, thereby weakening the adverse effect of water immersion on the mechanical properties of the central part of the coal pillar dam. Based on the analytic results of limit equilibrium theory and numerical simulation results, the reasonable width of the coal pillar dam was determined to be 40 m, corresponding to a central water-blocking zone width of 15 m. Engineering application results showed that the coal pillar dam with a width of 40 m had a smaller proportion of the plastic zone and lower horizontal deformation, meeting the stability requirements.