The problem of quantifying the effect of the alfalfa root morphology on the stability of the shallow surface layer of the slope of the Haizhou open-pit coal mine and the optimal slope degree in terms of the reinforcement of the shallow surface layer by the alfalfa root system was addressed. In this study, the mechanical parameters of plain soil and alfalfa root–soil composite samples were measured by indoor soil tests and triaxial compression tests, and a calculation model for the slope of the Haizhou open-pit coal mine was established in FLAC3D numerical simulation software to analyze the influence of the alfalfa root system on the maximum displacement of the shallow surface layer of the slope and the relationship with the fractal dimension of the alfalfa root system. The fractal dimension was applied to quantify the influence of the alfalfa root morphology to further investigate the relationship between the fractal dimension of the root system and the optimal slope of the shallow surface layer. The analysis showed that the fractal dimension of the alfalfa root system varied at different slope degrees, i.e., 40° > flat > 30° > 50°; the maximum soil displacement of the shallow surface layer of the slope increased with slope in nonlinear increments. Analysis of the fractal dimension of the alfalfa root system and the maximum displacement reduction rate at the different slope degrees revealed that the optimal slope degree of the shallow surface layer reinforced by alfalfa varied between 30° and 40°. The study results could provide a basis for further explaining the nature of the role of the alfalfa root morphology in reinforcing shallow surface soil and the optimal slope degree of the slope of the Haizhou open-pit coal mine reinforced by alfalfa roots.