In　order　to　investigate　the　defects　in　the　intersection　of　variable　polarity　plasma　arc　(VPPA)　welding　seam　with　the　friction　stir　welding　seam.　Different　kinds　of　specimens　with　crossing　and　overlapping　welding　seams　were　prepared.　Defects　in　the　overlapping　and　crossing　welding　seam　were　detected.　The　mechanism　of　defects　was　further　explored.　At　last,　the　schematic　of　welding　porosity　forming　were　given.