The　keyhole　digging　process　associated　with　variable　polarity　plasma　arc　(VPPA)　welding　remains　unclear,　resulting　in　poor　control　of　welding　stability.　The　VPPA　pressure　directly　determines　the　dynamics　of　the　keyhole　and　weld　pool　in　the　digging　process.　Here,　through　a　high　speed　camera,　high　frequency　pulsed　diode　laser　light　source　and　X-ray　transmission　imaging　system,　we　reveal　the　potential　physical　phenomenon　of　a　keyhole　weld　pool.　The　keyhole　depth　changes　periodically　corresponding　to　the　polarity　conversion　period　if　the　current　is　same　in　the　electrode　negative　(EN)　phase　and　electrode　positive　(EP)　phase.　There　exist　three　distinct　regimes　of　keyhole　and　weld　pool　behavior　in　the　whole　digging　process,　due　to　the　arc　pressure　attenuation　and　energy　accumulation　effect.　The　pressure　in　the　EP　phase　is　smaller　than　that　of　the　EN　phase,　causing　the　fluctuation　of　the　weld　pool　free　surface.　Based　on　the　influence　mechanism　of　energy　and　momentum　transaction,　the　arc　pressure　output　is　balanced　by　separately　adjusting　the　current　in　each　polarity.　Finally,　the　keyhole　fluctuation　during　the　digging　process　is　successfully　reduced　and　welding　stability　is　well　controlled.