In order to reveal the deterioration law of mechanical properties and the microscopic mechanism of slate under groundwater immersion, the macroscopic mechanical properties and microstructure evolution characteristics of slate under different immersion times were systematically analyzed by conventional triaxial compression tests and scanning electron microscope (SEM) tests. The results show that with the increase of immersion time, the failure mode of slate changes from shear failure to tensile failure, the peak stress and cohesion decrease in three stages of “fast-slow-stable”, the internal friction angle increases first and then decreases, and the confining pressure has a marginal decreasing effect on the strength maintenance. SEM test results show that the microstructure gradually develops from dense cementation (0 day) to loose cementation (30 days), fracture passivation and filling (60 days), and finally forms a penetrating fracture network (90 days). Macroscopic mechanical degradation is significantly related to the evolution of microstructure: cementation dissolution and crack propagation lead to a three-stage attenuation of strength, clay mineral expansion leads to a nonlinear change in the internal friction angle, and the development of a fracture network directly promotes the transformation of the failure mode. The research results provide a theoretical basis for the long-term stability evaluation of slate engineering under groundwater environment.