Abstract: CMOS high-speed camera was used to detect and analyze the transition behavior of aluminum alloy MIG welding droplets and to determine the transition frequency and speed of MIG droplets. Under the premise of theoretical analysis of MIG welding heat source characteristics, heat action mode and weld seam morphology, a combined volumetric heat source distribution model applicable to MIG welding was proposed and developed from the macroscopic welding thermal process. The MIG arc was treated and described as a classical double ellipsoidal heat source model, and the droplet energy action mode was represented as a uniform spherical heat source model. At the same time, the droplet thermal and kinetic energy are considered and added to the uniform sphere heat source model to realize the effect of droplet on the impact action of the MIG welding pool. Based on the above heat source model, a finite element model of the MIG welding temperature field of aluminum alloy was established. The temperature field of thick plate aluminum alloy MIG welding was simulated numerically. The results show that the trend and morphology of the weld fusion line obtained by simulation are in good agreement with the actual welding results. It is proved that the developed heat source model considering droplet thermal and kinetic energy can accurately describe the finger penetration characteristics and heat transfer process of MIG welding.