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In previous work, the low-power laser/arc hybrid welding technique was used to weld a magnesium alloy, and high-quality weld joints were obtained. For the pulsed laser and alternating current (ac) tungsten inert gas (TIG) arc, there are two typical phase matching modes between the laser pulse and the ac TIG arc: laser pulse acting on the positive wave, during which the tungsten electrode is the cathode, and laser pulse acting on the negative wave of ac TIG arc, during which the tungsten electrode is the anode. The welding plasma behaviors, welding penetrations, and weld bead appearances of the two phase matching modes are studied. The result shows that both the welding penetration and the weld appearance with laser pulse acting on the negative wave of TIG arc are better than those with laser pulse acting on the positive wave. The differences of the welding quality induced by the two phase matching modes are analyzed from the aspects of power input and spot force. Following this, the pulsed triggering and controlling system of pulsed laser and ac TIG arc was designed to control the phase matching mode in order to improve the welding quality in pulsed laser/ac TIG arc hybrid welding process.