Abstract: To investigate the evolution of surrounding rock stress and pore water pressure during the operation of underground reservoirs in abandoned mines, this study takes the Daliuta underground reservoir in the Shendong coal mining area as a case study. Considering different pumped storage volumes and frequencies, the dynamic variation patterns of the surrounding rock stress field and pore water pressure in the underground reservoir were analyzed using the FLAC3D numerical simulation method. The research results indicate that under the conditions of full−load continuous operation and conventional operation, the stress fluctuations in the surrounding rock of the underground reservoir are relatively small, with a maximum variation of no more than 0.5%, demonstrating overall structural stability. In contrast, the alternate−day pumped storage mode is more likely to induce stress concentration, posing a certain risk of local instability. The inner side of the coal pillar dam body is identified as a high−stress concentration zone, where the maximum principal stress reaches −18.32  MPa; however, the overall stress distribution remains relatively uniform. The pore water pressure in the dam body and surrounding rock exhibits a distinct periodic variation, mainly controlled by differences in rock permeability. Under the full−load operation condition, the pore pressure increases significantly, with a maximum rise of 32%, indicating that high−intensity pumping is a key factor causing abnormal pore pressure elevation. The displacement of the dam body is significantly influenced by the pumping mode and the thickness of the artificial dam, with the maximum displacement concentrated on the inner side of the coal pillar dam. The study suggests that moderate pumping intensity and stable operation rhythms help maintain the structural safety of the reservoir dam, and increasing dam thickness can enhance its deformation resistance.