The　crucial　factor　of　laser　welding　is　the　laser　energy　conversion.　For　a　better　understanding　of　the　process,　the　interaction　process　between　the　laser　beam　and　keyhole　wall　was　investigated　by　observing　the　keyhole　wall　evaporation　during　high-power　fiber　laser　welding.　The　results　show　that　the　evaporation　vapor,　induced　by　the　laser　beam,　discretely　distributed　on　the　keyhole　wall.　A　tiny　＇hollow＇　zone　was　observed　at　the　spot　center-action　region　on　the　FKW.　The　evaporation　vapor　induced　by　the　spot　center　moved　downward　along　the　front　keyhole　wall　(FKW)　with　a　period　of　about　0.3　similar　to　0.75　ms,　which　indicates　that　the　keyhole　formation　is　reminiscent　of　a　periodical　laser　drilling　process　on　the　FKW.　The　evaporation　vapor　on　the　keyhole　wall　suggest　the　assumption　that　the　laser　energy　coupling　mode　in　the　keyhole　was　multiple-reflection,　and　the　keyhole　depth　was　mainly　determined　by　the　drilling　behavior　induced　by　the　first　absorption　on　the　FKW.　(C)　2017　Optical　Society　of　America