The box girder of reinforced concrete structure is divided into precast box girder and cast-in-place box girder. The box girder prefabricated in independent site combined with bridge erecting machine can be erected after the completion of the lower project, which can speed up the progress of the project and save the construction period; Cast-in-place box girder is mostly used for large continuous bridges. Among them, steel box girder, also known as steel plate box girder, is a common structural form of long-span bridges. Generally used on bridges with large spans. It looks like a box, so it is called a steel box beam. Steel plate box girder is often used in engineering. In order to study the influence of diaphragm spacing on the distortion of simply supported steel box girder under concentrated load, the distortion effect and rigid torsion effect of simply supported steel box girder under concentrated load are compared, and the curve of the maximum distortion effect with the number of diaphragms is obtained. Concentrated load is applied to the top of box girder web, and the calculation is carried out by load decomposition method under four working conditions: distortion, rigid torsion, symmetrical bending and eccentric load.