A 20 mm-thick AZ31 Mg alloy plate was welded by friction stir welding, and the microstructure and mechanical properties of the joint were characterized in five layers along the thickness direction. The results showed that the grain size in the stir zone was uneven due to different welding temperatures and temperature drop rates. Different strong textures formed in different zones of the joint were mainly related to the material flow behavior. The yield strength and ultimate tensile strength (UTS) of the layered samples gradually increased from the top to the bottom surfaces of the joint. The elongation first increased from 9.5% to 10.5% and then decreased to 6.0% along the thickness of the joint. The UTS of the whole joint reached 88.5% that of the base material. The main strengthening mechanism of the joint was fine-grain strengthening.