Abstract: Under the same other welding conditions, friction stir lap welding of 5A06 aluminum alloy and AZ31 magnesium alloy was carried out by changing welding speed, and its influence on the formation of hook groove and tensile shearing property of the joints was analyzed. The results show that the heat input and flowability of plastic material are improved when the welding speed is reduced properly, and the effective lap thickness and width increase and the pore defects in the joint disapear. In addition, the interface between aluminum alloy and magnesium alloy near the hook groove is metallurgically bonded owing to sufficient atomic diffusion. However, when the welding speed is too low, the heat input increases dramatically and an obvious transition zone near the hook groove appears, and a large number of Al₁₂Mg₁₇ and Al₃Mg₂ brittle intermetallic compounds are formed in this zone, leading to many cracks. When the welding speed is 47.5mm/min, the largest effective lap thickness and width in the joint are obtained, and the interface in the front of the hook groove has a good metallurgical bonding, so the tensile shearing property of the joints is the best.