Tungsten inert gas(TIG) welding was performed on 2.7 mm thick commercial extruded AZ31 B magnesium alloy plates. We investigated the effect of post-weld heat treatment(PWHT) on the microstructure, mechanical properties and precipitated phase of the weld joints. The results showed that during the annealing treatment(200 ℃-1 h, 250 ℃-1 h, 300 ℃-1 h, 350 ℃-1 h, 400 ℃-1 h, and 450 ℃-1 h), the average grain size in the weld seam was the minimum after annealing at 400 ℃ for 1 hour, and then abnormally grew up after annealing at 450 ℃ for 1 hour. The mechanical properties enhanced when the joints were processed from 200 ℃-1 h to 400 ℃-1 h but sharply decreased with increasing annealing temperature. In contrast to the annealing treatment, solution treatment(250 ℃-10 h, 300 ℃-10 h, 350 ℃-10 h, 400 ℃-10 h, and 450 ℃-10 h) exhibited a better ductility but a slight deterioration in tensile strength. Especially speaking, no eutectic compounds(such as Mg17 Al12) were observed in the weld seam. The supersaturated Al atoms were precipitated in a coarse spherical shape dispersed in the weld seam. The precipitated Al atoms dissolved in the matrix substances at the condition(400 ℃-1 h) or(250 ℃-10 h). The solution treatment caused grain coarsening and precipitated Al atoms dissolved in the weld seam substantially, which resulted in a drop in micro-hardness at the weld seam compared to the area of the annealed joints. Tadium inert gas (TIG) welding was performed on 2.7 mm thick commercial extruded AZ31 B magnesium alloy plates. We investigated the effect of post-weld heat treatment (PWHT) on the microstructure, mechanical properties and precipitated phase of the weld joints. showed that during the annealing treatment (200 ℃ -1 h, 250 ℃ -1 h, 300 ℃ -1 h, 350 ℃ -1 h, 400 ℃ -1 h, and 450 ℃ -1 h), the average grain size in the weld seam was the minimum after annealing at 400 ° C for 1 hour, and then abnormally grew up after annealing at 450 ° C for 1 hour. The mechanical properties enhanced when the joints were processed from 200 ° C -1 h to 400 ° C -1 h In contrast to the annealing treatment, solution treatment (250 ° C for 10 h, 300 ° C for 10 h, 350 ° C for 10 h, 400 ° C for 10 h, and 450 ° C for 10 h) a better ductility but a slight deterioration in tensile strength. Especially speaking, no eutectic compounds (such as Mg17 Al12) were ob The supersaturated Al atoms were precipitated in a coarse spherical shape dispersed in the weld seam. The precipitated Al atoms dissolved in the matrix substances at the condition (400 ° C-1 h) or (250 ° C-10 h) . The solution treatment caused grain coarsening and precipitated Al atoms dissolved in the weld seam substantially, which resulted in a drop in micro-hardness at the weld seam compared to the area of ​​the annealed joints.