A laser-metal active gas welding（MAG） hybrid welding technology is used to weld Q345 B carbon steel. The effect of the main parameters（contain laser power,defocusing amount and distance between laser and MAG arc）on the welding stability and weld formation are took into account. And verify the feasibility of the hybrid welding technology for single-sided welding double-sided molding. The results show that when the filament spacing is the same,the laser-MAG is deeper than the MAG-laser in welding depth;when distance between laser and arc is less than 2 mm,the arc is stable,when it’s 2 mm,the weld penetration is the largest;when defocusing amount is 0 mm,the arc is the most stable,when it’s-2 mm,the weld penetration is the largest;when the current reaches more than 200 A,the arc has a significant effect on the shielding of the laser. In addition to verify the feasibility of laser arc welding in single-sided welding double-sided forming process.