The reinforced sandwich composite cantilever structure sees a wide application in the ship engineering. However, due to many design variables, it is diffcult to determine the bending stiffness at the design stage. Based on typical structure application background demands, a bending stiffness calculation model for the reinforced sandwich composite beam with variable cross-section is established. A model is made for stiffness tests, the result of which is compared with the calculation model. Finally, the influences of several factors on the stiffness are discussed, such as the material parameters of the main components, the hybrid fiber content of the panel, the allocation proportions of the panel and web sections, and the layer proportion of the web. The results can be used in designing reinforced sandwich composite cantilever structures.