Abstract: During additive friction stir deposition (AFSD) process, the plastic flow directly affects both macroscopic and microscopic forming as well as the microstructure and properties, but it is difficult to observe experimentally. A 3D thermomechanically coupled computational model of AFSD of CNT/Al composite material was established based on Coupled Eulerian-Lagrangian (CEL) algorithm of ABAQUS software. By comparing the measured temperature results of the additive manufacturing (AM) center point in the AFSD test of CNT/Al composite materials with the simulated results, the accuracy of the model was verified. The temperature field and equivalent plastic strain field during the AFSD process were analyzed in detail, and the material flow behavior was reflected using particle tracing technology. Additionally, the influence of different process parameters (rotation speed and advancing speed) on thermal deformation and material flow was explored. The results indicate that, compared to that of the advancing side, the temperature at the receding side is higher and the high-temperature area is broader, the distribution pattern of equivalent plastic strain corresponds to the temperature distribution. The material flow is most intense as the rod passes through, and the metal at the retreating side moves more vigorously compared to the advancing side. There is extensive material mixing between the substrate at the interface and the additive layer. Increasing the rotation speed and decreasing the travel speed both raise the peak temperature of the AFSD workpiece, leading to material softening and facilitating plastic deformation, thereby intensifying the extent and range of material flow and migration.