The invention discloses a method for improving the plasticity of an underwater stir friction welding joint and relates to a stir friction welding and modified treatment method for solving the problem that the plasticity of a heat-treatable strengthened aluminum alloy underwater stir friction welding joint is reduced. The method comprises the following steps of: 1, firmly positioning a welded workpiece and a stainless steel base plate by using a clamp, wherein the water surface in a water tank is 30mm higher than an upper end face of the welded workpiece; 2, performing stir friction welding by using a welding set along an butted surface of the welded workpiece to form a connecting welding line, wherein the welding parameters are the rotating speed of 800 revolutions per minute and the welding speed of 100mm per minute; and 3, retracting the welding set to an initial position of the connecting welding line, shifting to one side of the connecting welding line, wherein the one-time offset is equal to the width of the connecting welding line, performing one-time modified treatment from the initial position after one-time offset so as to obtain an modified welding line, wherein the direction for performing the one-time modified treatment is the same as the welding direction of the connecting welding line, and the rotating speed and the welding speed of the welding set are the same as those in the step 2. The method for improving the plasticity of the underwater stir friction welding joint is used for stir friction welding.