The invention provides a retractable friction stir welding spindle, aims at the deficiencies and defects of the existing friction stir welding spindle technology, that is, when the welding is finished, the stirring needle leaves a keyhole at the end of the weld seam, which affects the forming performance of the workpiece, the overall structure design of the spindle is complicated with many transmission chains and low reliability which is difficult to realize modularization. The retractable friction stir welding spindle includes an electric spindle system and a retracting mechanism. The electric spindle system includes stirring shaft shoulder, hilt, front end cover, hollow spindle, motorized spindle shell, motor rotor, motor stator, tail end cover, etc. The retracting mechanism includes stirring needle, connecting ball head and electric cylinder. The invention has the advantages of simple structure and can be mainly divided into two modules to improve reliability and integrity. The invention can also avoid the generation of friction stir welding keyhole.