Friction stir welding fixture in mechanical trigger control mode and method for controlling friction stir welding fixture

The invention discloses a friction stir welding fixture in a mechanical trigger control mode and a method for controlling the friction stir welding fixture. The friction stir welding fixture comprises a fixture base, a control unit, a trigger element, limit switches and a plurality of pressure plates; each pressure plate is connected with a pressure plate cylinder, each pressure plate cylinder corresponds to at least one limit switch and is controlled by the corresponding limit switches, and the limit switches and the pressure plate cylinders are connected with the control unit; and the trigger element and the limit switches are matched with one another and are mounted on a friction stir welding head. The pressure plate cylinders are controlled by the mechanical trigger type limit switches and the trigger element, possibility of malfunction of the pressure plate cylinders is eliminated, the pressure plates are driven by the pressure plate cylinders to be automatically opened before the welding head moves to t