Abstract: Frictional heat generated during the mixing of solid propellant slurry is a critical factor affecting process safety, as localized overheating may trigger premature curing or thermal decomposition. Based on the discrete element method (DEM), a three-dimensional geometric and kinematic model of a 1 L vertical kneader and its planetary impellers was established to numerically simulate the slurry mixing process. The DEM thermo-mechanical coupling module was adopted to analyze the conversion of mechanical energy into thermal energy, and the heat-distribution characteristics of the particle system and blade surfaces were monitored. The results show that the proposed model can effectively reproduce the slurry mixing process; the mixing uniformity can reach 41.98% and 89.68% at revolution angles of 100° and 600°, respectively, which verifies the rationality of the model and the mixing performance of the kneader. At the initial mixing stage, heat is concentrated as localized hot spots in the blade–particle contact regions, and then gradually spread to the kneading zone and the narrow-gap region between the outer blade and the vessel wall. High-temperature regions on the blade surfaces are mainly concentrated on the forward-moving faces, particularly at the kneading face where intense shearing and compression take place. Compared with the inner blade, the outer blade exhibits more pronounced heat accumulation owing to its higher relative velocity and more intensive particle–blade interactions, rendering it the primary heat source during the mixing process.