In order to investigate the deformation and mechanic property of the off-track simply supported steel boxgirder strengthened by diaphragms，by regarding the box-girder with diaphragms as a high-order statically-indeterminate structure of inner redundant connections，a distortional flexible method considering the secondary shear deformation of cross-section is proposed，and the iterative algorithm based on the precise time-integration method is adopted to improve the accuracy of the flexibility equation． Then，based on the superimposition principle，the solutions for the flexural displacement and stress of the simply supported steel box-girder with diaphragms are both obtained with the shear lag effect being considered，and the solutions for torsional warping functions is deduced． Moreover，an initial parameter method of distortion is put forward by taking into account the shear deformation． Finally，the analytical solutions for the displacements and stress of the box-girders under offset load are obtained． The results show that the position of diaphragms has a significant influence on the longitudinal and transversal distortional displacements but with only a little influence on the flexural and torsional displacements，and that the diaphragms can effectively increase the distortional frame rigidity of box girders and greatly reduce the transversal flexural deformations caused by the cross-sectional distortion．