Abstract: AC magnetic field was employed in welding of 5 mm thick AZ91 magnesium alloy plate by TIG method. The effects of post weld heat treatment on properties and microstructure of welded joints were studied through the analysis of the microstructure, mechanical properties and corrosion resistance. The results showed that the grain size was refined for AZ91 magnesium alloy joint with solution treatment; and more dispersed β-Mg₁₇Al₁₂ phase precipitates in α-Mg were found for the joint with solution and aging treatment; intermittent and dispersal precipitations of β-Mg₁₇Al₁₂ phase was smaller for joint with aging treatment. Meanwhile, the hardness, strength and plasticity of the weld joints were improved after heat treatment.