The invention provides a welding tool of a battery module. The welding tool comprises a top plate, a bottom plate and an extrusion mechanism. Supporting columns are arranged on the periphery of the upper surface of the bottom plate, the tops of the supporting columns are connected with the top plate, the extrusion mechanism comprises an air cylinder and an extrusion plate, the air cylinder is arranged on the center of the upper surface of the bottom plate, a piston of the air cylinder is connected to the center of the lower surface of the extrusion plate, guide pipes are arranged on the periphery of the air cylinder, guide columns are arranged in the guide pipes, the tops of the guide columns are connected to the lower surface of the extrusion plate, the bottoms of the guide columns extendto the lower portion of the bottom plate and are connected with fixed plates, two guide columns are connected on each fixed plate, connecting parts are arranged on the periphery of the upper surfaceof the extrusion plate, the upper portions of the connecting parts are connected with a positioning plate, the periphery of the upper surface of the positioning plate is provided with positioning bases, a square hole is formed in the middle of the top plate, a welding cover plate is arranged in the square hole, multiple welding holes are arranged in the welding cover plate in a uniform array manner, and the welding holes are arranged in corresponding to cell pole columns of the battery module. The welding efficiency and the welding quality are improved, and adaptation is good.