A 30 m span simply supported steel plate composite girder bridge with continuous deck was taken as the research object, in order to realize the standardized design of steel plate composite girder and achieve the purpose of further popularization and application. The current design code of steel plate composite girder bridge in China was combined, the cost of bridge and the steel consumption were taken as the objective function, the dimension layout parameters and the number of main beam were taken as the design variables, and the stress, deformation, local stability as well as standard construction requirements were taken as the constraint conditions. The optimization model of the section parameters of the composite steel plate girder was established based on the above conditions, and the genetic algorithm was used for optimization analysis. The optimization results showed that the proposed algorithm was stable, reliable and efficient, and there was a large space for optimization, depending on the structure section layout of the project. Under the condition of keeping the width of the bridge and the layout of the driveway unchanged, the cost of using the six-beam section was the least, which was reduced by about 13% compared with that of the original design. The steel consumption with four-beam section was the least, 123.69 kg/m², and about 27% of the steel consumption was saved compared with the original design. The optimization results can provide reference for the section design of medium span steel plate composite bridge. Economic analysis showed that, the economic values of the span height ratio of the section were 20?23, 18?21, 14?17, respectively, when the main beam spacing was about 2.3 m, 3.2 m and 4.8 m.