The invention discloses a flexible welding fixture for longitudinal arms of torsion beams. The flexible welding fixture comprises a base, wherein a first clamping mechanism, a second clamping mechanism and a flexible adjustment mechanism are sequentially arranged on the base from left to right; the first clamping mechanism comprises a chuck controlled by a cylinder and a circular base; the secondclamping mechanism comprises a square base and a rotatable press arm controlled by the cylinder; and the longitudinal arm is placed on the circular base, the square base and the flexible adjustment mechanism and fixed. According to the flexible welding fixture disclosed by the invention, the torsion beams with different wheel tread requirements, of the same vehicle type, can be welded by the samefixture, the torsion beams with the different wheel tread requirements are rapidly adjusted to corresponding length dimensions through the flexible adjustment mechanism during welding and welding forthe longitudinal arms of the torsion beams is carried out, so that the longitudinal arms achieve the requirements of the different wheel tread requirements while being welded to a torsion beam assembly.