Abstract: In order to study the failure mode and bearing capacity of the steel-concrete composite segment and to provide an optimization basis for the local construction of a rigid frame bridge with mixed beams, a finite element model was used to analyze the influence of the height change of the steel cell on the stress of the composite segment, and two scale models of the composite segment were designed and fabricated. The mechanical performance of the original and optimized composite segments under the coupling action of compression and shear were tested. The ultimate compressive bearing capacity and failure mode of the composite segment were obtained. The research shows that the height change of the steel cell has little effect on the ratio of the axial force borne by the rear bearing plate of the composite segment. The ultimate failure state of the originally designed composite segment is controlled by the steel girder transition section, and the safety reserve coefficient of the steel girder transition section and the composite segment is 3.30 and 6.65, respectively. The height of the steel cell is reduced by 1/3, and the thickness of the local steel plate is reduced by 1/3 in the optimized steel-concrete composite segment. Its ultimate bearing capacity is still higher than that of the steel girder transition section and the safety reserve coefficient is 5.58. When designing the steel-concrete composite segment of hybrid-girder rigid frame bridges, it is recommended to rationally reduce the height of the steel cell and change the U-shaped perforated shear plate to the O-shaped perforated shear plate to improve the out-of-plane restraint effect on filled concrete.