Abstract: 4 mm thickness plates of AZ31B alloy were welded by CO₂ laser.Three welding modes of deep penetration welding, heat conduction welding and unstable mode welding were found, and laser power was found to be the main factor for welding mode.The ratio of topside width and penetration of "deep and narrow segment" was proposed to characterize the weld shape, and the influencing factors of weld shape were investigated.Heat input was found to be the most important factor, while the laser power efficiency affected by welding velocity and the size of plasma-flume-affected zone affected by laser power could also influence the weld shape.When welds are fully penetrated, low laser power, high welding velocity and parameter combination of low laser power and welding velocity is favorable for weld of deep and narrow shape.