In　this　study,　the　16　mm　thick　aluminum　alloy　was　successfully　welded　using　variable　polarity　plasma　arc　(VPPA)　keyhole　welding.　The　microstructure　and　microhardness　distributions　of　the　weld　were　systematically　measured　along　its　thickness　and　width.　To　evaluate　tensile　strengths　along　the　weld　thickness,　tensile　specimens　were　divided　into　upper,　middle,　and　lower　sections.　The　surface　temperature　distribution　of　the　weld　pool,　along　with　the　temperature　profile　through　its　thickness,　was　measured　using　thermometers　and　thermocouples.　Numerical　simulations　were　employed　to　calculate　the　pressure　oscillations　along　the　keyhole　boundary　and　to　analyze　the　temperature　and　flow　field　distributions　of　the　VPPA.　The　findings　initially　revealed　a　homogeneous　and　refined　microstructure　distribution　within　the　weld　seam.　The　weld　zone　exhibited　higher　microhardness　compared　to　the　base　metal　(BM).　The　weld　demonstrated　a　tensile　strength　of　187.86　MPa　and　an　elongation　of　24.76%,　corresponding　to　95.57%　and　78.65%　of　the　BM,　respectively.　The　tensile　strengths　and　elongations　of　the　upper,　middle,　and　lower　sections　displayed　a　similar　distribution　to　that　of　the　entire　weld.　The　mechanism　underlying　the　microstructure　distribution　of　the　weld　was　significantly　influenced　by　the　temperature　distribution　of　the　weld　pool,　dynamic　pressure　oscillations　in　the　mushy　zone,　and　fluid　flow　of　the　weld　pool.　Specifically,　the　mechanism　whereby　pressure　oscillations　fracture　existing　nuclei　and　generate　multiple　new　nuclei　has　been　proposed　to　explain　the　refined　microstructure　distribution.　These　findings　provide　crucial　insights　for　optimizing　VPPA　keyhole　welding　technology,　ensuring　stable　and　high-quality　welding　of　thick　aluminum　alloys.