Focal　spot　shape　and　size,　beam　power　intensity　distribution,　convergence　angle　and　depth　of　focus　which　are　determined　by　beam　quality　and　focus　system　describe　the　characteristics　of　focal　beam.　The　influences　of　these　parameters　on　depth　and　width　of　weld　seam　are　studied　in　theory　and　experiment.　Results　shown　that　enough　power　intensity　is　the　precondition　of　realizing　laser　penetration　welding.　Depth　and　width　of　weld　seam　rely　on　energy　penetrating　into　materials.　With　the　same　processing　parameters,　focal　spot　size　decides　the　acquired　power　intensity,　depth　of　focus　decides　the　alignment　distance　and　convergence　angle　has　effect　on　Fresnel　absorption　in　keyhol　wall.　All　in　all,　beam-focusing　characteristics　have　effect　on　results　of　laser　welding　with　different　energy　acquired　at　different　focus　beam　cross　section.　In　reality　application,　laser　system,　focus　system　and　focal　spot　position　should　be　selected　according　to　the　practical　requirements　in　order　to　acquire　ideal　quality　of　processing.