Abstract: Long-term exposure to water-rich environments can induce progressive damage and deterioration of rock structures. Therefore, investigating the mechanical properties and deterioration behavior of surrounding rock subjected to water invasion is of considerable significance for ensuring safe production in water-rich mines. In this study, uniaxial compression tests were conducted on sandy mudstone specimens subjected to different water invasion times using a mechanical testing system combined with an acoustic emission (AE) monitoring system. The mechanical properties, damage evolution, and instability precursor characteristics of sandy mudstone were systematically investigated at water invasion times of 0, 14, 28, 42, 56, and 70 d. The results indicate that the stress-strain curves of water-invaded sandy mudstone exhibit a four-stage evolution pattern. With increasing water invasion time, the failure behavior of the specimens gradually changes from brittle failure to ductile failure, while both the peak stress and elastic modulus exhibit nonlinear decreasing trends. After 70 d of water invasion, the compressive strength and elastic modulus decrease by 58.24% and 64.43%, respectively, compared with those of specimens in the natural moisture state. The cumulative acoustic emission ringing count initially increases and subsequently decreases with increasing water invasion time, reflecting the damage evolution characteristics of sandy mudstone during deformation and failure. These results further reveal a damage deterioration mechanism in which water invasion progressively promotes the transition from microstructural damage to macroscopic mechanical degradation. Moreover, the average AE b-value gradually increases with increasing water invasion time, rising from 2.11 in the natural moisture state to 3.01 after 70 d of water invasion. This variation indicates that the time-dependent effect of water invasion significantly influences both the scale and number of internal cracks in sandy mudstone. In addition, a sustained decrease in the b-value over a short period can be regarded as a potential precursor to rock instability and failure. The findings provide theoretical support and practical guidance for stability control and disaster early warning of coal seam floors under water-rich conditions.