In　order　to　reveal　the　fluctuation　mechanism　of　the　plume　induced　during　fiber　laser　keyhole　welding,　dynamic　behaviors　of　both　the　plume　and　the　keyhole　were　simultaneously　observed　by　using　the　multiple-imaging　method.　The　results　show　that　the　fluctuation　period　of　the　plume　was　the　same　as　that　of　the　laser-induced　vapor　moving　down　on　the　keyhole　front　wall　(KFW).　The　tilt　angle　of　the　plume　was　the　largest　when　the　laser-induced　vapor　was　close　to　the　orifice.　As　the　laser-induced　vapor　moved　toward　the　bottom　of　the　KFW,　the　tilt　angle　of　the　plume　decreased　gradually.　The　correlation　between　the　plume　fluctuation　and　the　dynamics　of　the　laser-induced　vapor　on　the　KFW　was　not　affected　by　parameters　such　as　welding　speed,　laser　power,　and　focal　length.　Further　analysis　shows　that　the　periodic　drilling　behavior　of　the　laser　spot　on　the　KFW　is　the　main　reason　for　the　periodic　fluctuation　of　the　plume.