The　dynamic　plume　behavior　is　one　of　the　key　features　that　allows　one　to　obtain　the　required　information　for　monitoring　the　process　quality　of　the　intelligent　laser　welding.　This　paper　proposed　a　method　of　directly　measuring　particle　velocities　in　plume　induced　in　fiber　laser　keyhole　welding　based　on　the　self-mixing　interferometry　(SMI)　effect.　This　method　was　verified　by　measuring　tiny　metal　objects　with　known　sizes　and　velocities.　The　particle　velocities　can　be　calculated　according　to　the　number　of　SMI　fringes　induced　by　the　particles,　and　the　temporal　data.　To　measure　the　diameters　of　particles,　it　is　expedient　to　use　a　detection　laser　with　a　spot　diameter　larger　than　that　of　particles.　A　smaller　diameter　of　the　detection　laser　spot　will　result　in　more　accurately　measured　values　of　particle　velocities.　A　direct　measurement　was　conducted　to　assess　the　particle　velocities　in　the　plume　induced　in　fiber　laser　keyhole　welding.　These　particles　in　plume　had　a　velocity　of　1.9-90　m/s,　and　more　than　45%　of　these　particles　had　a　velocity　of　10-60　m/s.　Particles　with　larger　diameters　were　more　likely　to　have　higher　velocities.