The　experiments　were　carried　out　with　CO2　(slab)　laser.　The　five　stages　of　the　inter　actions　between　laser　and　powder　in　the　laser　welding　with　powder　were　analyzed.　The　influences　of　the　filler　metal　powder　on　the　laser　power　density　threshold　value,　weld　formation　and　process　stability　were　studied　during　CO2　laser　welding　of　aluminum　alloy.　The　experiment　results　demonstrate　that　the　addition　of　the　metal　powder　enhances　the　energy　coupling　efficiency　and　reduces　the　power　density　threshold　value　during　laser　deep　welding.　Such　as　the　reducing　rate　is　about　50%　for　A2219　alloy.　The　experiment　results　also　indicate　that　the　powder　can　enter　the　weld　pool　availably　only　the　powder　was　delivened　to　the　suitable　area　after　laser　beam　at　the　surface　of　a　part.　The　proper　quantity　of　the　alloy　powder　must　match　with　the　right　power　density　and　energy　input,　which　can　assure　to　obtain　good　weld　appearance.　Meanwhile,　the　addition　of　the　powder　can　reduce　the　fluctuation　of　the　laser-induced　plasma　and　maintain　its　stability　effectively　in　the　laser　welding.　So　a　stable　welding　process　and　good　weld　appearance　can　be　obtained　in　the　experiments.