Abstract: The paper used orthogonal experiment to optimize the friction stir spot welding welding process of of Q235 steel and 6082 aluminum alloy plate. Results showd that when the Q235 steel plate was placed on the backward sidethe-metal at the steel side and near the stir pin was softened, and then short nail was formed under the shear stress induced by transverse-moving shoulder,ultimately the softened metal filled in the formed cavity at the rear of the stir by stir spinning. A pressed depth of 0.2 mm is favorable to form high-quality welds. When the rotation speed is 260 RPM, welding speed is 21 mm/min and pin offset is 0.2 mm towards aluminum alloy plat side, the tensile strength of the weld is 141.204 MPa, fracture occurs at the interface zone between the weld nugget area and the heat affected zone. The hardness of the heat affected zone at steel side was higher than parent metal while that near the aluminum alloy plat side was lower than the parent metal.