Abstract: Water inrush disasters in deep rock mass engineering result from the coupled actions of hydrostatic pressure, in-situ stress, and repetitive dynamic loads from blasting. In order to study the dynamic fatigue mechanical properties of rock under these conditions, cyclic impact tests were performed on red sandstone with varying water pressures and axial static stresses using an independently developed high water pressure and high stress rock dynamics testing system. The evolution of dynamic stress-strain curves and the laws of peak stress degradation were analyzed, and a degradation model for the rock’s dynamic peak stress under hydro-mechanical coupling was constructed. By establishing an empirical relationship between the dynamic peak stress and the average strain rate, the study investigated the degradation effect of the peak stress on the strain rate during the cyclic impact process and explored the underlying degradation mechanisms. The results showed that the dynamic peak rock stress decreased exponentially with the number of cyclic impacts. At a constant water pressure, the total number of cyclic impacts was negatively correlated with the axial static stress; conversely, at a constant axial static stress, the total number of cyclic impacts increased with increasing water pressure. A negative linear correlation was observed between the dynamic peak stress and the average strain rate. The absolute value of the dynamic peak stress degradation coefficient initially increased and then decreased with axial static stress at a fixed water pressure, whereas it remained relatively insensitive to water pressure at a fixed axial static stress. During cyclic impact loading, water pressure primarily exerted weakening effects such as the “water wedge effect” at crack tips, as well as strengthening effects composed of the confining effect of external water pressure, the Stefan effect, and the Meniscus effect. The competition between these two effects collectively governed the dynamic fatigue mechanical properties of the rock.