Single shoulder friction stir welding was used to achieve a 40 mm thick AZ31 magnesium alloy doublesided butt welding, the microstructure and mechanical properties of the joint were analyzed. The joint can be divided into five zones: base material (BM) zone, heat affected zone (HAZ), thermal-mechanical affected zone (TMAZ), nugget zone (NZ) and overlapped zone (OZ), of which the OZ is unique to Ds–FSW joints. The existence of ultra-fine grains and diffusely distributed precipitated phase Al₈Mn₅ in the OZ significantly increases the ultimate tensile strength (UTS) and elongation (EI) in this zone, respectively 217.7 MPa and 14.65%, resulting in a high joint efficiency ratio of about 85.81% of the base material. The joints are mainly fractured at the junction of NZ and TMAZ on the advancing side, and their fracture mechanism is a mixture of ductile fracture and quasi-cleavage fracture.