Abstract: Paste backfill in mine goafs is subjected to prolonged chloride attack and exposure to pressurized water, and its mechanical properties and structural stability are adversely affected by these coupled effects. To investigate the characteristics and mechanisms of mechanical deterioration under coupled water pressure and chloride attack, paste backfill specimens are immersed in a chloride solution for 90 and 150 d at pressures of 0, 0.5, 1.5, and 3.0 MPa using a custom-developed pressurized immersion apparatus. Eight groups of uniaxial compression tests are conducted with a three-dimensional full-field strain measurement system and an acoustic emission (AE) monitoring system. The strength, deformation and failure behavior, and AE characteristics are examined, and the underlying deterioration mechanisms are elucidated.The results indicate that a decrease in the mean uniaxial compressive strength is observed with increasing immersion duration. Repeated intermittent increases in AE ring-down counts are recorded, and damage accumulation is intensified by prolonged immersion. With increasing immersion pressure, the earlier onset of deformation localization bands and more pronounced stress concentrations are observed. AE signals are predominantly recorded during the pre-peak stage, whereas the maximum AE energy rate is predominantly recorded during the post-peak stage. With increasing immersion duration, the spread of surface pitting from the central region toward the specimen edges is observed, accompanied by an increase in the surface damage ratio. With increasing immersion pressure, a transition from fine, discrete pits to sheet-like spalling is observed, together with an increase in the surface damage ratio.Under coupled water pressure and chloride attack, microcrack development is promoted by surface tractions induced by the movement of the pressurized chloride solution within the specimens. Meanwhile, Friedel’s salt is formed through reactions of chloride ions with solid constituents and hydration products in the paste backfill. At low levels of Friedel’s salt formation, the pore structure is refined, and specimen compactness and strength are increased. With further accumulation of Friedel’s salt, pore development is promoted by crystallization-induced expansion, and the macroscopic mechanical strength is reduced. These findings provide a basis for understanding the stability of paste backfill and improving the control of surface subsidence.