Abstract: To address the issue of insufficient strength in thick aluminum alloy welding, a variable polarity plasma arc-tungsten inert gas(VPPA-TIG) composite welding process was proposed. This study further validates the welding quality of VPPA-TIG composite welding by conducting welding experiments on 2219 aluminum alloy plates with thickness of 17 mm, followed by a detailed analysis of the weld seam microstructure and mechanical properties. The mechanism of strength distribution characteristics is revealed by microstructure analysis. The results show that VPPA-TIG composite welding process produces high-quality weld seam formations with a superior degree of metallurgical bonding. The hardness distribution in the weld seam fusion zone is uniform, with an average value of 67.6±2.7 HV0.2. The tensile strength reaches 62.83% of the base material, while the elongation after fracture is only 24.8%. The increased thickness of the weldment expands the heat dissipation area of the weld pool, resulting in insufficient grain growth within the weld seam. This leads to the formation of "meandering" micropores in the interface region. These micropores will continue to grow when subjected to tensile loads, which is the main reason for affecting the tensile strength of the weld seam. This study further expands the application range of VPPA-TIG composite welding process, clarifies the mechanism of strength failure in thick plate weld joints, and provides a theoretical and technological reference for solving the welding of large-thickness plates.