This paper describes the features and characteristics of plasma induced in the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. In the air environment, the identified spectra are atomic lines of Al, Mg, Cr, Mn, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectrum of AlO, MgO and AlH. It has been confirmed that the resonant lines of Al and Mg were strongly self-absorbed. These facts have shown that the laser-induced plasma is relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO are different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum is predominant in intensity, while the AlO spectrum became much stronger with the increase in high power density. This was attributed by the great difference in boiling point and vaporization energy of Al and Mg. In argon atmosphere the band spectra of MgO and AlO completely vanished, but AlH molecular spectra is detected clearly. The hydrogen source is presumably the hydrogen solved in the base metal, absorbed water on the surface oxide layer, or H2 and H2O in the shielding gas.