The invention discloses a press tool for straightening welding deformation of a small-cross-section H-shaped steel flange plate. Multiple left rib plates and right rib plates are vertically welded to an operating platform. The operating platform is connected with a long-strip L-shaped clamping plate through the multiple left rib plates. The vertex angle of the right angle of the L-shaped clamping plate is connected with the inner side of H-shaped steel in an abutting-against manner, so that the H-shaped steel flange plate is straightened. Spaces for containing the H-shaped steel are formed between the left rib plates and the operating platform as well as between the lower portion of the L-shaped clamping plate and the operating platform. An end plate is arranged on the right rib plates. The end plate is connected with the inner side of the H-shaped steel in an abutting-against manner. Multiple support plates are arranged on the right rib plates. The upper ends of the support plates are fixedly connected with the end plate, and the lower ends of the support plates are connected with the operating platform. A round hole is formed in the middle of each support plate. Nuts are welded to the two side faces of each round hole. A lead screw is connected with a line-shaped clamping plate through the nuts, and furthermore, the line-shaped clamping plate is connected with the H-shaped steel in a fastened manner. The press tool is stable in structure and reasonable in design; and by means of the press tool, efficiency can be improved, and quality can be guaranteed.