The invention discloses a welding method for medium steel plates for boilers. The method includes the steps that 1, the thickness of the steel plates is 60-80 mm, the mode of connecting the steel plates with the same thickness in a butted mode is adopted, and at least three half-round grooves are formed in corresponding positions of two butted medium steel plates to be welded and form round grooves in a butted mode; 2, pre-welding is carried out, the two medium steel plates to be welded are preheated, and then the two ends of the two medium steel plates are welded so that the corresponding half-round grooves can be connected firmly; and 3, welding is carried out, the zigzag welding mode of welding from one end of the butt joint surface to the other end and then to the start end is adopted, back chipping is carried out on the back surfaces of butt welding connection points, shielded metal arc welding is adopted in welding and finally post heat treatment is carried out. According to the method, to-be-welded surfaces of the two medium steel plates are divided into at least four sections through the half-round grooves, then deformation of non-thermal effect areas of the medium steel plates is reduced to the minimum degree through spaced zigzag welding, and therefore the mechanical property of all areas of welded joints can be guaranteed to be qualified.