In this paper, a method of stress-strain relation of concrete under triaxial loading is proposed. The method is based on an orthotropic model of three dimensional equivalent increment. Bathe's approach of the stiffness matrix is used for reference. The failure criterion for concrete by Ottosen and the peak strain criterion by Ahmad are adopted in the model. Compared with the experimental data by Richart, the theoretical results are found to have a high accuracy. Another distinguished advantage of the approach is that only a few material parameters are needed to be determined when the method is applied.