In this paper, a detailed design of double-line continuous U-beam bridge with variable width of urban rail transit is carried out. This bridge make a three-span continuous structure, it have a main span of 45m. With the width of 11.01m at the small mileage and 14.96m at the large mileage, it have a variable width far greater than that of other bridges of the same type, so the difficulty of the design process increases rapidly. The design combines the continuous structure system with the standard U-beam, it greatly reduces the structural volume, and forms a lightweight landscape together with the station. The design of the mid-web variable width segment adopts the system design method of “web thickens +box added + web forked”, which the structure changes from three webs to four webs during the process of broaden its width. The whole bridge is so complex and needs further study. By establishing the beam element model and three dimensional finite element model of the whole bridge, the transverse stress analysis, the local stresses of the variable width segment, the support reactions and multiple web shears distribution and the overall spatial stress effect analysis are carried out. The stress mechanism of the bridge is further discussed. Through the optimization analysis of the whole bridge, the thickness of the web and bottom plate of the bridge is reduced to further improve the mechanical performance of the structure, which provides a rational design method for this type of bridge.