To further improve the numerical calculation of variable-polarity plasma arc (VPPA)–metal inert gas (MIG) welding, the heating behavior of VPPA-MIG welding was analyzed using a high-speed camera. A variable-polarity finite element model was developed and optimized for the numerical analysis of the thermal process in VPPA–MIG welding. This model incorporated the heating mode of the VPPA with its periodic variations. Moreover, in order to more accurately express the hybrid welding conditions, the coupling interaction between the two heat sources in the electrode-negative and electrode-positive phases was taken into account. Experiments were conducted to obtain weld dimensions to verify the predicted results. This simulation results showed that the fusion width of the VPPA–MIG welded joint was smaller than that of the MIG welded joint, and the penetration ability of hybrid welding was also stronger. There was a conspicuous difference in the form of heat input and thermal process between VPPA-MIG and MIG welding, which directly affected the microstructure and mechanical properties of the welds. Comparing the microstructures of the VPPA-MIG and the traditional MIG welds, it was found that the swing of the hybrid arc in VPPA-MIG welding can facilitate the production of fine aluminum grains in thick aluminum alloy plates. Consequently, the hardness and tensile properties of the hybrid welded joints were significantly higher than those produced under MIG traditional welding.