The invention discloses a pipe fitting plastic forming three-dimensional principal stress theory forming limit diagram manufacture method. The method comprises the steps that the pipe fitting is cut and manufactured to one-way tensile specimens, and a tensile test is conducted on the one-way tensile specimens to obtain basic parameters of the pipe fitting; on the basis of the basic parameters, a first ultimate principal stress and a second ultimate principal stress are obtained in different thickness direction stress conditions; according to a preset third principal stress, the first ultimate principal stress and the second ultimate principal stress, the three-dimensional principal stress theory forming limit diagram is drawn. According to the manufacture method, the basic parameters of the pipe fitting are obtained through the one-way tensile test, so that according to the basic parameters of the pipe fitting, the first ultimate principal stress and the second ultimate principal stress are obtained, and according to the first ultimate principal stress, the second ultimate principal stress and the preset third principal stress, the three-dimensional principal stress theory forming limit diagram is drawn; by introducing the thickness direction stress, the three-dimensional principal stress theory forming limit diagram is formed, and therefore the accuracy of pipe fitting forming limit prediction is improved.