<p>The current stability of large Quaternary deposit slopes on the Tibetan Plateau remains uncertain and may be affected by nearby engineering activities, potentially imposing constraints on construction projects. To determine the formation mechanism and current stability of an important large Quaternary deposit slope located 200&#xa0;m from a planned important engineering project, field investigations, discrete element simulations, and parameter inversion methods were employed. The studied deposit slope originated from multiple phyllite collapses, which contributed to this deposit and shifted the river channel from right to left. Terrestrial laser scanning provided a precise predeposit three-dimensional terrain model for the formation simulation, and the simulated deposit slope was validated in terms of overview and profile characteristics. Furthermore, the formation process and stability assessment were connected via repose angle experiments to overcome the difficulty in experimentally determining the shear parameters of large Quaternary deposit slopes. Finally, the three-dimensional limit equilibrium method was used to assess the stability of this important deposit slope, where the factor of safety in all working conditions exceeded 1.0. The analysis methods and conclusions provide a scientific reference for engineering construction and research concerning large Quaternary deposit slopes.</p>