The 3 mm thickness industrial pure aluminum and AZ31 magnesium alloy dissimilar metals joints were successfully manufactured by friction stir welding（FSW）. The influence of post-weld annealing treatment on microstructures and mechanical properties of dissimilar Al-Mg friction stir welded joint were investigated in detail. Using OM,EDS,IMCs deformability testing,tensile properties testing and SEM for fracture surface analysis. The results showed that the interface IMCs have obvious growth after annealing treatment. The interface IMCs layer cannot cooperate deformation with the both sides of the matrix metal,almost no deformation ability;the tensile strength of the as-welded FSW Al-Mg dissimilar joints was 106 MPa,92% of the base metal（BM）of aluminum.Meanwhile,the elongation was 3.2%,91% of BM of Al. The tensile strength and elongation of the Al-Mg joints decreased with the IMC grown up during the process of thermal of 300 ℃ with the holding time of 20 min,40 min and 60 min,which were 94 MPa/1.3%,70 MPa/0.7% and 28 MPa/0.4%,respectively. Accordingly,the fracture mode changed from ductile fracture to brittle fracture.