To solve the safety problem of using waste slurry to backfill discarded open pit mine, the treatment method of vacuum preloading at bottom combined with upper surcharge loading is implemented in the model test. The feasibility of using this method in the backfilling of discarded open pit mine is discussed. The test results show that the proposed method can significantly reduce the water content of the slurry and increase the shear strength of the soil. The water content of the slurry reduces from 450% to 95%–105%, and the reduction of volume reaches 73.4%. The undrained shear strength increases from zero to 9.8–13.4 kPa. In the initial self-weight consolidation stage, there is gravitational separation in the deposition of particles, and the deposition of coarse particles at the bottom is helpful to alleviate the clogging problem of vacuum preloading. During the stage of vacuum preloading, seepage direction of pore water in the slurry is not completely one-dimensional downward, and there is a hydraulic gradient in the radial direction. After the treatment, the compressibility of the soil is close to that of soft clay, while the permeability is greater than that of soft clay. Based on the experimental results and the large-strain consolidation theory, the effects of slurry thickness of a single disposal cycle on volume reduction and treatment time are analyzed using the finite difference method with consideration of the self-weight, nonlinear change of compressibility and permeability of the slurry. The process parameters for operation in site are recommended based on the results of numerical analysis.