Hybrid laser-arc welding was used to join AZ31B Mg alloy. Weld penetration, surface formation were researched, and effect of arc stability was investigated in positive and negative welding directions during hybrid welding. The results showed that the weld penetration was increased by more than about one times and the weld stability increased by more than one times, compared the positive direction hybrid welding with gas tungsten arc welding only. It has been proved that the welding for magnesium is a perfect process. For negative direction hybrid welding, welding stable was further improved, but weld penetration was not increased. In addition, arc stability in hybrid welding and arc root location were discussed.