Abstract: The steel-ultrahigh performance concrete (UHPC) composite box deck system has a good application prospect in the construction of long-span bridges. Based on an actual bridge, two large-scale steel-UHPC composite box girder tests were conducted mainly to study the overall bending performance of the steel-UHPC composite box girders in the elastic stage. We focused on the deflection and strain distribution of the specimens. An analytical model of the composite box girder considering the shear lag, slip effect and shear deformation of the steel web was established and the analytical solution was derived. The results show that the shear lag effect was significant under concentrated loads. The ratio of the maximum to minimum values of the UHPC slab mid-span compressive strain was 5~7. With the increase in the UHPC slab depth, the stiffness of the steel-UHPC composite box girder increased and the strains on both the UHPC slab and steel girder decreased. Compared with the model without considering the shear deformation of the steel web, the analytical solution in this paper was generally in good agreement with the test results.