The invention discloses an arc assisted laser welding method. Bifocal laser adopted by the method is formed by dividing a single laser beam into a bifocal laser beam through an optical beam splitting system, double laser beam focuses vertically distributed in the weld joint direction can do mutual crisscross motion, a molten pool is fully and effectively stirred, when the bifocal laser with high power is used as a welding heat source, the weld penetration is advantageously increased, the molten pool is stirred, pores and splashing are effectively reduced, and meanwhile, the weld joint weld penetration and weld width can be regulated and controlled by finely adjusting the focal distance of the two focuses. When the bifocal laser with low power is used as a heat treatment heat source, the smooth finish of welded surfaces is advantageously improved, the weld joint uniformity is improved, pores and splashing are effectively reduced, and the weld joint quality is improved. According to the method, laser energy is dominant, low-power single-focus laser and arc are adopted for pre-welding heat treatment of workpieces, through plasma interaction between the arc and the bifocal laser heat source, the absorptivity of laser energy is increased, the laser keyhole effect is improved, and thus the larger weld penetration or higher welding speed can be achieved.