Compound　tracking　was　proposed　to　obtain　the　flow　trajectory　of　aluminum　keyhole　molten　pool　in　a　successful　and　high　quality　welded　joint.　The　Cu　element　was　selected　to　form　an　easily　distinguishable　compound　with　Al　matrix.　Al-Cu　distributions　at　each　interface　were　obtained　by　slicing;　3D　reconstruction　was　used　to　describe　the　flow　behavior　in　the　keyhole.　The　results　showed　that　material　flow　in　the　leading　side　flowed　to　the　rear-side　along　the　keyhole　wall　without　complex　convection.　It　became　complicated　before　keyhole　closing　phase.　During　weld　formation,　the　metal　in　the　center　flowed　toward　the　surface　and　the　bottom,　while　the　metal　in　the　wall　flowed　toward　the　center　(The　center　is　the　middle　area　along　the　thickness　direction　of　the　keyhole　rear　side).　Material　flow　during　the　entire　variable　polarity　plasma　arc　(VPPA)　process　can　be　clearly　explained　by　this　method.