Weld　surface　roughness　is　an　important　criterion　for　evaluating　weld　quality.　The　main　factors　that　affect　weld　surface　roughness　in　fiber　laser　keyhole　welding　are　identified　by　an　in　situ　observation　of　the　dynamic　behavior　of　a　keyhole　mouth　and　by　analyzing　the　eruption　characteristics　of　the　laser-induced　vapor　on　the　front　keyhole　wall　(FKW).　In　fiber　laser　keyhole　welding,　the　weld　surface　roughness　first　decreases　and　then　increases　with　the　increase　in　welding　speed.　Both　the　distortion　rate　of　the　keyhole　mouth　and　its　range　of　fluctuation　will　increase　as　the　welding　speed　increases.　The　fluctuation　in　the　distortion　rate　of　the　keyhole　mouth　diameter　can　reflect　the　stability　of　fiber　laser　keyhole　welding.　The　laser-induced　vapor　at　the　FKW　can　erupt　to　impact　the　rear　keyhole　wall,　and　this　is　the　main　factor　that　affects　the　distortion　rate　of　keyhole　mouth　diameter　and　weld　surface　roughness　in　fiber　laser　keyhole　welding.　In　fiber　laser-arc　hybrid　welding　or　in　CO2　laser　keyhole　welding,　plasma　can　suppress　the　effect　of　laser-induced　vapor　impacting　the　keyhole　and　reduce　the　weld　surface　roughness.