The invention belongs to the technical field of mechanical equipment manufacture and relates to a special welding tool for a motor train chair backrest. The main body structure of the special welding tool comprises first small rib plates, cross beam base plates, second bottom corner base plates, vertical beam base plates, second small rib plates and third small rib plates, wherein the two right-angle triangular third small rib plates are fixedly arranged at the positions which are close to the middle part of the upper edge of a bottom plate for supporting and limiting the lower side of a first cross beam of the motor train chair backrest; and the right-angle triangular second small rib plates are separately arranged at the positions which are close to the upper parts of the left and right side edges of a through hole for supporting and limiting the outer side of a second cross beam. The main body structure is simple, the design concept is skillful, small area is occupied, labor force is saved, the work efficiency is increased, and corresponding tools can be adjusted according to required chair backrest sizes and dimensions so as to meet the requirements of various backrest chairs. The special welding tool is great in practicability and convenient to mount and use, and at the same time, is friendly to application environments and wide in market prospects.