Abstract: [Objective] Paleogene red beds are widely distributed in the Xining Basin. Clarifying the deformation and failure characteristics of red beds under water immersion conditions, as well as their underlying mechanisms, can provide theoretical support for the prevention and control of geological hazards such as collapse and landslide in red-bed areas, as well as for related engineering construction. As one of the main lithological components of the red beds in this region, the gypsified sandstone of the Mahalagou Formation plays a crucial role in controlling the stability of red-bed slopes. [Methods] In this study, gypsified sandstone from the Mahalagou Formation was selected as the research object. Triaxial compression tests under different soaking durations were conducted to analyze the effects of soaking time and confining pressure on the deformation and failure characteristics of the gypsified sandstone. Combined with scanning electron microscopy (SEM) observations and ion composition analysis of the soaking solution, the material loss characteristics of the gypsified sandstone under immersion conditions were identified, and the deterioration mechanism of its mechanical properties was further discussed. [Results] The results show that, with increasing soaking time, the failure mode of the gypsified sandstone exhibits a significant transition from brittleness to ductility, while the compressive strength and elastic modulus both show a marked attenuation trend, accompanied by a continuous extension of the plastic deformation stage. The cohesion decreases exponentially with increasing soaking time, whereas the internal friction angle shows only slight fluctuations overall. The failure mode of the gypsified sandstone is jointly controlled by soaking time and confining pressure. The dried specimens are dominated by single shear failure, which gradually evolves into tensile-shear composite failure and even tensile failure after soaking. Although confining pressure can inhibit the propagation of oblique primary cracks and enhance the plastic deformation capacity of the gypsified sandstone, it cannot reverse the water-induced deterioration process. Analysis of the ion composition of the soaking solution confirms that cementing materials such as calcium sulfate in the gypsified sandstone undergo dissolution and loss. SEM observations further indicate that the dissolution of the gypsified sandstone is the key deterioration factor responsible for the sharp decline in its mechanical strength. [Conclusion] These findings reveal the water-induced deterioration characteristics of gypsified sandstone in the Mahalagou Formation and provide a theoretical basis for the prevention and control of geological hazards on red-bed slopes in the Xining Basin, as well as for the design and construction of related infrastructure projects.