Laser　beam　wavelength,　incidence　angle,　polarization　and　it＇s　time　and　space　behavior　are　the　main　factors　concerned　with　laser　manufacturing.　Laser　beam　quality　quantifies　the　space　behavior.　The　effect　of　beam　quality　on　laser　penetration　welding　based　on　the　focusing　theory　of　multimode　beam　is　studied　through　the　measurement　and　calculation　the　beam　quality　of　two　different　high　power　laser　processing　systems　and　the　analysis　the　results　of　laser　penetration　welding　experiments.　The　results　show　that　due　to　the　effect　of　laser　beam　quality　on　focal　beam　waist　radius,　focal　angle　and　focal　length,　laser　beam　quality　decides　the　beam　focusing　ability　and　the　processing　capacity　of　laser　system.　Furthermore,　beam　quality　should　be　take　into　account　when　selecting　the　focusing　system　and　the　position　of　focal　point.