Abstract: In order to study the heat and mass transfer and molten pool flow characteristics of aluminum alloy by cold metal transition (CMT) and arc additive manufacturing, a three-dimensional numerical model for CMT arc additive manufacturing was established based on Fluent software. In the model, dynamic meshing technology is used to simulate the reciprocating motion of welding wires in the vertical direction. The volume of fluid method is used to capture the gas-liquid interface. The enthalpy-porosity method is used to track the solid-liquid interface, periodic heat input and stage arc force are applied to equivalent arc discharge behavior. The heat and mass transfer process of welding bead formation and the dynamic behavior of the molten pool are studied and analyzed. The results show that the residual height and slope of the weld pool are relatively large, and the morphology is like a half sphere in the early stage of weld bead forming. In the later stage of forming, the accumulation of heat causes the residual height behind the weld bead to be slightly smaller than the front, while the rear end melting width being slightly wider than the front end. During the single droplet transition period, the mechanical withdrawal of welding wire has the most significant impact on the surface flow of the molten pool, and the liquid bridge fracture produces a significant recoil effect on the molten pool. The electromagnetic force acts as the dominant driving force inside the molten pool to generate a clockwise circulation, which continuously strengthens and weakens with the cycle switching of the arc burning stage, and basically runs through the entire transition period, making the thermal convection inside the molten pool more complete. The simulation results are in good agreement with the experimental results.