The　process　of　YAG　laser　and　MIG-arc　hybrid　welding　for　5A90　aluminium-lithium　alloy　was　investigated　based　on　analysing　the　characteristics　of　joint　formation　in　laser-arc　hybrid　welding.　Under　the　condition　of　the　constant　welding　position,　the　effect　of　main　welding　parameters　on　the　joint　formation,　including　laser　power,　travel　speed,　welding　current　and　the　distance　between　hybrid　thermal　sources,　was　discussed.　The　results　indicate　that　a　good　synergistic　effect　is　shown　if　the　distance　between　laser　and　arc　is　within　0-6　mm.　The　laser　power　plays　a　main　role　in　increasing　of　depth　and　backside　width　of　weld.　With　the　increase　of　welding　speed,　the　width　of　the　weld　decreases　rapidly,　so　dos　the　penetration.　But　the　depth　and　width　of　the　weld　increase　obviously　with　the　promotion　of　welding　current.