The preset invention discloses a welder of a square multi-leg stirrup welding device for shear walls. The welder comprises a lower welding electrode and an upper welding electrode, and is characterized in that the lower welding electrode is formed with a vertical run-through mounting hole between two electrode tips. The welder also comprises a magnetic member and a lifting cylinder. The lifting cylinder is vertically fixedly installed in the mounting hole, with a piston rod upward. The magnetic member is fixedly installed at the upper end of the piston rod; the rear end face of the magnetic member is a vertical plane parallel to the array direction of the two electrode tips of the lower welding electrode. As the magnetic member extends out along with the piston rod of the lifting cylinder, the upper end of the rear end face of the magnetic member is capable of rising beyond the upper end faces of the two electrode tips of the lower welding electrode and forming a magnetic adsorption portion. After the piston rod moves back, the upper end of the magnetic member is capable of descending below the upper end faces of the two electrode tips of the lower welding electrode. The welder is capable of limiting legs, more accurately controlling the landing points of the legs, and better ensuring the smooth welding between the legs and the rectangular reinforcing steel bars.