To analyze the flexibility effect of shearing deformation on cantilever box girder of sandwich composite materials，the equivalent bending stiffness and shear stiffness of the box girder are deduced，the cantilever box girder is equivalent to an isotropy box girder which has the same long-span and stiffness. The flexibility which takes shear deformation into account of sandwich composite materials of cantilever box girder is analyzed by primary beam theory. The effect of shear deformation to the whole flexibility of the box girder is analyzed. The methods and conclusions are available for the designers on box girder of sandwich composite materials.