This　study　investigates　the　underlying　causes　of　Variable　Polarity　Plasma　Arc　Welding　(VPPAW)　spatial　positional　instability　and　its　adverse　impact　on　weld　seam　quality.　Microstructural　analysis　reveals　that　the　primary　cause　of　poor　transverse　mechanical　properties　is　the　asymmetric　distribution　of　grains.　Through　the　study　of　heat　and　mass　transfer　in　the　molten　pool,　it　was　found　that　the　asymmetric　flow　of　molten　metal　under　the　influence　of　gravity　is　the　main　factor　leading　to　uneven　temperature　distribution　and　asymmetric　grain　distribution　in　the　weld　pool.　A　novel　approach　based　on　a　predefined　temperature　field　is　proposed　to　regulate　the　weld　pool＇s　stability　and　welding　quality.　The　implementation　of　the　predefined　temperature　field　effectively　improves　VPPAW　weld　pool　flow,　leading　to　enhanced　temperature　distribution　and　grain　size　and　distribution.　These　findings　provide　a　theoretical　foundation　and　valuable　engineering　recommendations　for　achieving　high-quality　spatial　position　welding　in　VPPAW.　©　2023　The　Author(s)