The invention relates to a
friction stir welding method, in particular to a double-side symmetric
friction stir welding method, which solves the technical problems of asymmetric structure and mechanical properties, root defects, difficulty in clamping, easiness in breakage of a self-sustaining
friction stir welding stirring pin, and low
welding efficiency and joint property of a single-side friction stir
welding joint. The method includes: firstly, fixing workpieces to be welded onto a worktable and positioning the workpieces on the same horizontal plane; secondly, plunging two stirring heads into the two workpieces respectively at the same rotation speed
omega in the same or opposite rotation directions to reach preset positions; and thirdly,
welding the workpieces to be welded by the aid of the two stirring heads at the same welding speed. By means of adjusting the rotation directions of an upper shaft shoulder and a lower shaft shoulder, flow of materials can be improved, functional
asymmetry of heat engines on two sides of the cross section of a joint is eliminated, the
mechanical property of the joint is improved, the two stirring heads are used for acting on the upper surfaces and the lower surfaces of the workpieces to be welded respectively to avoid the root defects, and the two stirring heads simultaneously generate heat so that welding efficiency can be remarkably improved.