According to the sectional deformation of single-box double-cell box girder,some displacement parameters are designed,and the displacement modes of horizontal and vertical distortion angles of the cross-section are deduced.Then,the calculation diagrams of diaphragms and side walls are simulated by using rigid frames or plates,and both the shear strain energy and the shear stiffness of diaphragms and side walls are deduced based on the principles of mechanics and finite element.Finally,the proposed method is compared with the commercial finite element software.It is found that the proposed theory and method are both correct and complete,that the proposed method is superior to the commercial finite element software with similar calculation results,and that the shear strain energy of diaphragms and side walls increases with the displacement by a power function.In addition,the optimal opening position of diaphragms of single-box double-cell box girder is determined in this paper.