To study the mechanical properties of the steel-concrete composite section between steel box girders and steel-ultra-high performance concrete (UHPC) composite girders, the Guanyinsi Yangtze River Bridge with a main span of 1 160 m was used as the background. Through refined finite element modeling, the stress characteristics under the most unfavorable load conditions and transmission mechanism of the steel-concrete composite section were analyzed, and the main factors affecting the stress of the composite section were discussed. The results indicate that under the most unfavorable load conditions, the stress of each component in the steel-concrete composite section is within 160.4 MPa, which is lower than the material strength design value, and there is sufficient safety reserve. Along the longitudinal direction of the bridge, the influence of the composite section on the force transmission of the bottom plate and middle web of the steel box girder is relatively small, and the stress generally shows a downward trend from the steel box girder to the composite girder direction, with a variation amplitude within 20 MPa. The stress levels of the top plate, web plate, and bottom plate of the steel lattice chamber decrease by 25.6%, 73.3%, and 35% near the bearing plate, respectively. In the process of transmission force through shear nails, the proportion of force transmitted by shear nails of the top plate in the steel lattice chamber is the highest, and a closer proximity to the end of the composite girder indicates a higher proportion, with a maximum of 57%. Increasing the thickness of concrete filled in the composite section can reduce the structural stress, and a thickness of 350 mm–400 mm can achieve the optimal stress of the structure. A longer composite section is more conducive to the stiffness transition of the shear nails.