Abstract: In order to overcome the problem of easily unstable molten pool in thick plate aluminum alloys perforation welding, this study started from arc temperature field during thick plate welding, aiming to explore mechanism of molten pool stability of thick plate piercing welding. By establishing a three-dimensional numerical model, temperature field, flow field and current density distribution of variable polarity plasma arc (VPPA) with 5 mm and 16 mm thickness of welding base metal were studied. The results showed that with the increase of plate thickness, temperature field distribution of VPPA changed. Arc temperature decay of 5 mm plate thickness was linear, while that of 16 mm plate welding was quadratic, which made temperature change more complicated. When welding 5 mm thick aluminum alloy plates by VPPAW, current density of the entire keyhole molten pool was above 1.0 × 10⁶ A/m², but when welding 16 mm thick aluminum alloy plates, the same current density only reached 12 mm depth of the small hole. This study was expected to provide theoretical guidance for realizing stable welding of large thickness aluminum alloy and broadening application range of VPPAW.