Abstract: The friction stir welding (FSW) process tests of three-layer composite plate Al/AZ31/Al at different welding speeds were carried out, and the joint forming, microstructure and tensile properties were observed and analyzed. The results show that under the optimized process parameters, the welded joint is well formed, and its internal distribution is layered, and no defects are found. The grain refinement in nugget zone (NZ) is obvious, and the large angle grain boundaries (HAGBs) and recrystallized grains account for 80%. There is intermetallic compound (IMC) in the banded structure area (BS) on the forward side of the weld and the interface between magnesium and aluminum, which is mainly composed of Al3Mg2 and Al12Mg17. With the increase of welding speed, the tensile strength of welded joint first increases and then decreases. When v=100 mm/min, the average grain size of the aluminum layer in the weld nugget area is 1.75 μm, and the tensile strength of the welded joint reaches the maximum 87.3 MPa, which is 50.8% of the base metal.