Slope stability analysis is a popular area of interest in geotechnical engineering and is significant to landslide prevention and other geohazards. Owing to the success of the combined numerical methods of Lagrange and Euler, the material point method (MPM) has been developed. The benefit of MPM for the study of the evolution process for the slope is its application to efficiently simulate small- to large-scale deformation processes. The high-performance computing GPU-based MPM code named as CoSim-MPM is developed and the strength reduction method (SRM) is introduced to perform the quantitative analysis of slope failure risk assessment. Based on the developed code, a method to define the factor of stability (Fos) is provided and validated for two typical slopes: a homogenous slope and a slope with a weak interlayer. By comparing the MPM results with that of the limit equilibrium method (LEM), it is shown that both of the potential slide surfaces and Fos correspond well with each other. The accuracy of the MPM-SRM results surpasses that of the LEM for the failure mechanical and deformation characteristics of the landslide. As a practical application, the stability and failure mechanisms of Dongmiaojia slope, downstream of the Xiaolangdi Dam, China, is analyzed. All the results show that the GPU-based MPM can have powerful advantages for the stability analysis of slope with high efficiency. In addition, CoSim-MPM has strong scalability and can be applied to large-scale simulation research in the field of geotechnical engineering.