In order to study the design approach of the inner mold holder for the cantilever pouring construction of box girder with complex cross-section and the stress characteristics of the construction stage, relying on the Sila Murong Bridge, the design approach of the inner mold holder suitable for the side chamber and the middle chamber of the box girder is proposed, and the corresponding cantilever pouring process of the box girders with complex cross-sections is improved. The finite element model of the inner mold holder is established by Midas Civil software, and the mechanical characteristics of the inner mold holder are analyzed under the condition of considering the load during construction, and the feasibility of the proposed design approach of the inner mold holder is verified by combining the material strength analysis and stability analysis. The results show that the stress level of the members in the center area of the side chamber holder and the middle chamber holder is significantly higher than that in the edge area, so it is suggested that the cross-sectional size or infill arrangement of the members in the center area should be appropriately increased to improve the bearing capacity when the inner mold holder is optimized. In order to further improve the construction safety, the arrangement of the middle room holder beam can be appropriately enhanced. At the same time, if conditions permit, it is recommended to minimize the influence of vertical deformation of the holder on the accuracy of the vertical mold by preloading the holder. The results can provide a reference for the design and optimization of the inner mold holder in the cantilever pouring construction of box girders with complex cross-sections.