Abstract: In order to study the flexural behavior of a steel-ultra high performance concrete (UHPC)-normal concrete (NC) composite girder, carried out were the flexural tests for two steel-UHPC-NC composite girders with different UHPC-NC interfacial treatment approaches and one comparative steel-NC composite girder, and investigated were the failure modes, interface slip, flexural capacity and deformation, and strain regulations of tested girders. Considering the cohesion contact relationship of UHPC-NC interface, a refined numerical model was constructed through finite element program and utilized for the parameter analysis, in which the effect of interfacial reinforcement ratio and interfacial cohesion on flexural behaviors were examined. According to the flexural behavior and failure characteristics of test girders, proposed was an analytical method of interface shear resistance at the UHPC-NC interface for steel-UHPC-NC composite girders. The results show that steel-UHPC-NC composite girders have good cooperative working performances among all parts under flexural loading and dramatically fail in a plastic flexural mode at their final failure. The flexural capacity of steel-UHPC-NC composite girders is higher than that of steel-NC composite girders due to the ultra-high mechanical properties of UHPC layer on the top surface with the same section size. The UHPC-NC interface adopting the treatment method of roughening by chiseling and strengthening by sufficient shear reinforcement can reliably ensure that the UHPC-NC composite slab always works together to achieve a full combination action. The flexural properties obtained from the numerical model is consistent with the test, thusly, the numerical model can be used to calculate and analyze the whole flexural process of steel-UHPC-NC composite girders. The shear resistance results of UHPC-NC interface in terms of parameters in AASHTO LRFD specifications and of the UHPC-NC interface shear calculation method proposed are basically consistent with those of the test girders, which is more suitable for the UHPC-NC interface shear analysis of steel UHPC-NC composite girders.