In　this　study,　the　self-mixing　interferometry　(SMI)　method　was　used　for　in　situ　measuring　the　micron-sized　particles　characterization　in　the　focused　fiber　laser　beam　(FFLB)　during　the　fiber　laser　keyhole　welding.　The　micron-sized　particles　in　the　FFLB　have　a　diameter　of　0.655-6.55　mu　m　and　a　velocity　of　1-90　m/s,　and　particles　with　a　larger　diameter　exhibited　higher　velocities.　The　share　of　particles　with　a　diameter　of　0.655-2　mu　m　and　that　of　particles　with　a　velocity　of　1-10　m/s　decreased　with　the　laser　power,　and　increase　with　the　detection　height.　The　number,　diameter　and　percentage　of　particles　with　higher　velocities　increased　with　the　laser　power　and　decreased　with　the　detection　height.　These　trends　were　due　to　particle　vaporization　induced　by　laser　in　the　FFLB.　In　situ　measurements　characterization　of　particles　was　found　instrumental　in　adjusting　the　laser　power　and　detection　positions.