Steel box girder, also called steel plate box girder, is a common structural form for long-span bridges. It is generally used on bridges with larger spans. It is called steel box girder because it looks like a box. Steel plate box girder is a commonly used structural form in engineering. In order to study the influence of the spacing of diaphragms on the distortion of simply supported steel box girders under concentrated loads, simply supported steel box girders with different numbers of diaphragms are set up, and the distortion effect and rigid torsion effect under concentrated loads are compared to obtain the maximum distortion effect curve with the number of diaphragms. A concentrated load is applied to the top of the box girder web, and the load decomposition method is used to calculate according to four working conditions: distortion, rigid torsion, symmetrical bending and eccentric load.