A　bright　spot　(laser　beam　spot-center-induced　metal　vapor)　on　the　keyhole　front　wall　(KFW)　was　observed　during　fiber　laser　keyhole　welding　of　a　＇sandwich＇　specimen　to　reveal　the　interaction　between　the　laser　beam　and　the　KFW.　The　dynamic　behavior　of　the　bright　spot　shows　that　the　keyhole　formation　is　the　result　of　periodical　drilling　of　the　laser　beam　spot　center　on　the　KFW　during　welding.　Welding　parameters　have　little　effect　on　the　drilling　period.　The　drilling　speed　first　increases　and　then　decreases　with　the　increase　in　the　keyhole　depth,　and　the　next　drilling　process　begins　before　the　drilling　speed　reduces　to　0　m　s(-1).　The　average　drilling　speed　can　be　increased　by　increasing　the　line　energy　or　power　density　of　the　laser.