Abstract: Aluminum alloy A6061 and magnesium alloy AZ31B were welded using resistance spot welding. According to the characteristics of dissimilar metal welding, tin metal layer was added during the welding process to control the joint performance. The microstructure of the joint was observed by scanning electron microscope and the properties were tested by the tensile machine. The effects of three main process parameters (welding current, time and electrode pressure) on the morphology of nuggets and mechanical properties of A6061/Sn/AZ31B joints were studied. The results show that Al₁₂Mg₁₇ columnar crystal layer (about 10 μm) is formed at the nugget boundary area of magnesium alloy, and Al₃Mg₂ and eutectic structure layer of Al (about 3.5 μm) are formed at the nugget boundary area of aluminum alloy. With the increase of the three parameters, the shear load of A6061/Sn/AZ31B joint increases firstly, then reaches the maximum value, and then decreases. When the welding current is 30 kA, the welding time is 20 cyc, and the electrode pressure is 2.1 kN, the shear load of the joint reaches the maximum of about 4.6 kN.