Both the increasingly-thin walls of box girder cross-sections and the super-loads from trains or track vehicles make the behavior research on diaphragms and side walls become particularly important. According to the deformation of single cell box girder cross-section, the displacement parameters were designed, and the displacement modes of cross-section distortion angles were deduced. The schematic plans of diaphragms and side walls were simulated by rigid frames or plates. Both the shearing strain energy and the shearing stiffness of diaphragms and side walls were deduced on the basis of the principles of mechanics and finite element. The results show that the shearing strain energy increases with the displacement by a power function. And the method calculation results are close to the commercial FE software calculation results, which show that the calculation method of single cell box girder presented in this paper is correct and reliable. The calculation method is simpler than the commercial FE software calculation method. In the calculation method, whether diaphragms and side walls are calculated separately or calculated as some parts of the box girder or the vehicle - bridge system, the calculation precision of diaphragms and side walls remains unchanged.