Some qualitative factors influencing the stability of rock debris slopes are difficult to be quantified using limit equilibrium and numerical analysis methods. Quantification theory can take full consideration about the influence of qualitative factors and can be used to quantify them. Based on the quantification theory, the stability assessment model of rock debris slopes are presented. According to the material composes of rock debris slopes and the contact condition of inner particles, the structural classification of rock debris slopes are presented. Based on the main factors analysis, four evaluation indexes are proposed to establish the stability assessment model. The four indexes are (a) the saturated uniaxial compressive rock strength (divided into four levels), (b) the structure of rock debris (divided into three levels), (c) the slope of ground surface (divided into three levels), and (d) the conditions of groundwater (divided into two levels). The application of the model to Yunnan Shuifu-Maliuwan highway acquired a sort of predicted values. They have a good agreement with the measured values. The model can be used for the stability assessment of similar projects.