Welding　trials　of　autogenous　laser　welding,　laser　welding　with　filling　wire　and　laser-MIG　arc　hybrid　welding　have　been　carried　out.　The　characteristics,　cause　and　controlling　methods　of　joint　defects　were　analyzed　by　optic　microscope,　X-ray　detection　and　numerical　simulation.　The　results　indicated　that　the　common　defects　were　imperfect　shape　and　porosity　inside　the　weld.　The　characteristics　of　weld　surface　defects　varied　during　different　welding　processes;　porosity　was　also　different　in　shape　and　formation　during　full　penetration　welding　and　incomplete　penetration　welding.　All　the　defects　mentioned　above　could　be　controlled　or　avoided　by　optimizing　the　welding　process　procedures.