The utility model discloses a universal joint yoke boring and milling head with 90 degree extension arm for boring and milling machines, comprising an extension arm cylinder, an intermediate driving shaft and a milling cutter spindle. The utility model is characterized in that the front end of the extension arm cylinder is fixed with a box body of a support head, and the intermediate driving shaft is arranged inside the extension arm cylinder and is supported by the method that the end parts of the front side and the back side are sheathed with bearings. The front part of the intermediate driving shaft penetrates the inner of the box body of a support head, and the front end is provided with a bevel wheel. The milling cutter spindle is arranged inside the box body of the support head and is arranged with the intermediate driving shaft in 90 degrees. One end of the milling cutter spindle penetrates out of the box body of a support head, and the penetrating end is provided with a discal cutter. The milling cutter spindle is supported by the bearings sheathed on the end parts of the both sides of the milling cutter spindle. The milling cutter spindle is provided with a bevel wheel which is engaged with the bevel wheel installed on the front end of the extension arm. The utility model can become the special process tool which can process the wedge sliding surface of the support device of the upper part of the converter body with 250 ton steel, and can carry out the milling process to the four wedge sliding surfaces of two converters with 250 ton steel, which can reach and be superior to the design requirement.