To reveal the influence of slotting pressure relief parameters on the failure behavior and floor heave control effect of soft rock roadway floors, laboratory biaxial compression tests combined with digital image correlation (DIC) technology were conducted. The deformation and crack evolution processes of sandstone specimens under varying slot widths and depths were investigated, and the mesoscopic damage mechanism was analyzed using the PFC2D discrete element method. The results indicate that during the loading process, the sandstone specimens undergo four stages: compaction, elastic deformation, plastic deformation, and failure. Although slotting slightly reduces the peak strength, it significantly enhances the post-peak ductility. DIC monitoring reveals that the high-strain zones in unslotted specimens are initially scattered and gradually propagate into the deep rock mass. In contrast, the pressure relief slot utilizes the tip effect to induce localized strain concentration at the slot bottom, achieving directional energy release and transfer. In unslotted specimens, accompanied by numerous tensile cracks, shear cracks interconnect to form a macroscopic slip surface, manifesting as a deep penetrating failure. Conversely, the slotted specimens transform into a localized wedge failure originating from the slot bottom; crack activity is restricted to the vicinity of the slot, thereby effectively maintaining the structural integrity of the deep rock mass. Parameter sensitivity analysis shows that the slot width primarily regulates the deformation release space, while the depth controls the truncation effect of the stress transmission path. Within the scope of this study, the slotting combination of 7 mm in width and 17 mm in depth exhibited the optimal floor heave control effect. The findings can provide a reference for understanding the floor heave mechanism in soft rock roadways and optimizing the parameters of slotting pressure relief.