Abstract: Based on the Yunnan Honghe bridge, two segmental model tests of a steel-UHPC composite box girder were designed. The research focused on bearing capacity, on failure mode and on crack resistance of the composite box girder. The shell element-based finite element models (FEM) were established in ABAQUS and verified by test results. Parameter analysis was carried out to study the influence of UHPC slab thickness, reinforcement ratio, stud spacing and steel beam diaphragm thickness on the mechanical performance of the composite box girder. The design suggestions for each parameter were developed finally. The results show that: the steel-UHPC composite box girders exhibit high bearing capacity and good crack resistance; considering the limitation of maximum crack width, the bearing capacity of the 180 mm-thick equivalent UHPC bridge deck is more than 8 times of the vehicle load standard value; the thickness of UHPC slab and steel beam diaphragm show a great influence on the failure mode of the composite box girder; considering the balance of economy and mechanical performance, the actual thickness of UHPC bridge deck should be 210 mm and below, the reinforcement ratio should be 1.4% and below, and the stud spacing should be 450 mm and below.