AZ31 magnesium alloy sheets were welded by CO2 laser welding with filler metal process. The effect of different amount of calcium (Ca) additions on the AZ31 laser welded joints was studiesd, and its effect on the microstructure and mechanical properties was investigated. In the fusion zone, the grains were refined with an appropriate amount of Ca addition (no more than 0.19wt%Ca), resulting in the improved mechanical properties of the joints. The optimum welds were obtained with the AZC04 filler metal strip (0.19wt%Ca contained in the fusion zone), the grains and the precipitates in the fusion zone were both refined, and the welding efficiency of the joints was up to 100% for the base metal.