Abstract: Friction stir welding was performed on a lock bottom structure (groove depth of 6 mm) composed of an annealed fork ring and a quenched and artificial aged short shell of 2A14 aluminum alloy. The influence of pin length (4.0-7.0 mm) on microstructure and properties of the joint was studied. The results show that incomplete penetration defects existed at the root of the joint when the pin length was less than 6.0 mm, and the “Hook” defect appeared on the edge of the weld on the side of the short shell when the pin lenght was no less than 6.0 mm. With increasing pin length, the height of the “Hook” increased. With increasing pin length, the grain size of the weld nugget zone decreased first and then increased, and the tensile properties increased first and then decreased. When the pin length was 6.0 mm, the grain size of the weld nugget zone was the smallest, the hardness was the highest and the tensile properties was the best. When the pin length was 4.0 mm, the tensile fracture of the joint showed delamination, and the joint had a mixed fracture mode of quasi-cleavage and dimple fracture. When the pin length was no less than 6.0 mm, the joint showed ductile fracture.