Abstract: Friction stir spot welding of 2mm thick plates of A6061 were conducted using the cylindrical tool with left-hand thread, the influence of plunge depth of shoulder and rotational speed of tool on interface migration was investigated. The reason of interface migration was analyzed by the plastic flow in the plate thickness. The mechanical properties of the welding spot were investigated by interface migration. The results showed that when the plunge depth of shoulder or rotational speed increased, the lap interface in two sides of the key-hole moved upwards to the top of the welding point and far way from the center of the key-hole gradually. As a result, the effective bonded width increased and the effective plate thickness decreased. The ring plastic flow in the plate thickness was the reason for lap interface migration. When the plunge depth of shoulder or rotational speed increased, the tensile shear load increased firstly and then decreased. When the effective bonded width was small, the welding spot was fractured in model Ⅰ. When the effective bonded width was large, the welding spot was fractured in model Ⅱ.