In this study, a detailed design for a double-line continuous U-beam bridge with a variable urban rail transit width is produced. This bridge has a three-span continuous structure and a variable width far greater than that of other bridges of the same type, which means that the difficulty of the design process increases rapidly. The design of the variable width segment adopts the “web thickened + box added + web forked” system design method for the mid-web. The entire bridge is complex and requires further study. Through establishment of a beam element model and three-dimensional finite element model of the entire bridge, analysis of the overall spatial stress effect, transverse stress, local stresses of the variable width segment, and support reactions and multiple web shear distributions are carried out. The stress mechanism of the bridge is then further discussed. Through optimization analysis of the entire bridge, the thicknesses of the web and bottom plate of the bridge are reduced to further improve the mechanical performance of the structure. Therefore, the method used in this study can be used to rationally research and design this type of bridge.