Abstract: Three kinds of welding seam cross-sectional pattern can be found in laser-arc hybrid welding of magnesium alloy. According to the influence of welding parameters on the patterns, the interactive mechanism was proposed. By investigating the atom distributions in the arc plasma, the plasma spectral information and laser keyhole behavior, the mechanism was verified as well. Results show that the effect of laser on arc plasma is realized by both the laser keyhole and the plasma in the keyhole.At certain proper parameters, the arc plasma collumn can extend into the lase keyhole, and discharge with the keyhole plasma. A hybrid plasma can be formed finally. The coupling of arc plasma and keyhole plasma in this style can improve the energy density of the heat source.