The encased concrete in a composite bridge with corrugated steel web (CSW) is a critical structure to improve the stability of the web and relieve the stiffness difference between the composite girder and intermediate crossbeam. This study aims to investigate the effects of the encased concrete on the mechanical behaviors of composite girder bridges with CSWs. The finite element model of a rigid frame bridge is established, the calculation method of the shear force sharing ratio of the encased concrete is proposed, and the effect of the length and thickness of the encased concrete is analyzed. The results show that the encased concrete was under compression in longitudinal direction, while vertical tensile stresses occur near the transition section between pure CSW and the composite web. The encased concrete shares 65–90% of the total shear force, and the shear force sharing ratio increases with the increase of the concrete thickness. Increasing the length of the encased concrete reduces the deflection of the girder and pre-compression stress of the top concrete slab. All the findings of present study may provide reference for the design of composite girder bridges with CSWs.