In　the　process　of　variable　polarity　plasma　arc　welding　(VPPAW),　the　state　of　the　keyhole,　particularly　the　back-side　keyhole,　was　a　critical　factor　influencing　the　weld　formation　quality.　However,　in　practical　welding　operations,　monitoring　the　keyhole　state　from　the　backside　with　sensors　presented　significant　challenges.　Therefore,　this　paper　proposed　a　method　for　detecting　the　back-side　keyhole　on　the　front　view　using　plasma　arc　welding　based　on　the　reflection　from　the　front　wall　of　the　molten　pool.　With　appropriate　shooting　angles,　this　method　could　simultaneously　capture　both　the　front-side　keyhole　images　and　the　reflected　shape　of　the　back-side　keyhole　on　the　front　wall.　The　relationship　between　the　welding　quality　and　the　reflective　shape　of　the　keyhole　was　established　through　experiments　at　different　welding　currents.　With　the　increase　of　welding　current,　the　position　of　the　detected　keyhole　image　on　the　front　wall　was　gradually　shifted　upward.　When　the　weld　appeared　to　be　cut,　the　reflective　shape　of　the　keyhole　on　the　back　disappeared;　when　the　stable　weld　state　was　disturbed,　the　reflective　shape　of　the　keyhole　on　the　back　changed　at　the　same　time.　The　stability　of　the　keyhole　was　directly　related　to　the　stability　of　the　welding　states.　The　reflective　shape　of　the　backside　keyhole　on　the　front　wall　was　affected　by　the　welding　state　and　the　molten　pool　state.　This　provided　important　guidance　for　establishing　the　correlation　between　the　reflective　shape　of　the　keyhole　on　the　back　and　the　welding　quality.　©　2025　Harbin　Research　Institute　of　Welding.　All　rights　reserved.