Belonging to the field of weld ultrasonic automatic inspection, the invention in particular relates to a pressure vessel cylinder circumferential weld ultrasonic automatic inspection device. A lifting motor, a lifting speed reducer and a gear pair are arranged on a connecting plate, the lifting motor drives the lifting speed reducer on the upper side of it, the gear pair falls into a left gear and a right gear, the lifting speed reducer drives the left gear to rotate, the left gear drives the right gear to rotate, the right gear cooperates with a lead screw and drives the lead screw to rotate, and a nut is connected to the lead screw, the lead screw rotates and makes the nut move up and down in the lead screw, the nut is connected to a nut seat, and a scanning device has cylinder scanning function, i.e. lifting scanning movement along a pressure vessel axis and extension movement along the pressure vessel radial direction, when bottom head scanning is carried out, a locking air cylinder drives a locking pin to get out of a locking groove, so that a swing shaft can have the swing function, and bottom head circumferential weld ultrasonic non-destructive detection can be realized.