This paper clarified the key problems in the application of the friction stir welding technology to the lock circumferential weld of the tank. Firstly, the conventional structure of the lock joint used to fusion welding must be redesigned, and the back welding groove and the front welding groove were eliminated. The results showed that the mechanical properties of the lock joint presented the best performance when the length of the pin was equal to the thickness of the short shell, and the fork ring was located on the advancing side of the weld. The results of ultrasonic phased array analysis showed that the signal display existing at the interface of assembly interface and welding nugget can not be interpreted as the defect signal. The vertical assembly welding scheme is used to assemble and weld the lock circumferential weld, and a set of tooling is developed to assure the assembly clearance, the wrong side, the flatness of the weld, and so on. The friction stir welding technology has been applied to the lock joint of the Φ3350 mm tank has been realised, and its nondestructive test results and geometrical dimension met the design requirements.