The invention relates to a method for straightening deformation of a tank opening of a large transformer tank body, and belongs to the technical field of steel structure welding. According to the technical scheme, reinforcing iron (2) on the deforming portion of the tank opening (1) of the transformer tank body is heated, a flame heating portion is placed on the reinforcing iron, the front face heating area (3) of the reinforcing iron is of a rectangular shape, two side face heating areas (4) of the reinforcing iron are of an isosceles triangle shape, the length of the bottom edges of isosceles triangles is the same as the width of the rectangular front face heating area, and the front face heating area and the two side face heating areas form the heating part of the reinforcing iron together. The heating part of the reinforcing iron is placed at the position, 2/3-4/5 of the length of the reinforcing iron, on the reinforcing iron from the end of the tank opening. The heating temperature of the reinforcing iron is 500 DEG C - 600 DEG C. The work of straightening deformation of the tank opening of the large transformer tank body is achieved after heating. The method for straightening deformation of the tank opening of the large transformer tank body has the advantages of being simple and practicable in method, low in cost, high in efficiency, and capable of not influencing the performance such as the strength of the tank body after straightening is finished.