Abstract: The composite hull is mainly composed of longitudinally or transversely stiffened laminates. The design of composite stiffened laminates can be approximated as an orthogonal anisotropic plate and the equivalent stiffness of stiffened composite plates needs to be calculated quantitatively. The longitudinal and transverse bending stiffness of hat-stiffened panel were derived using the Castigliano's theorem. The analytical expressions for the equivalent bending stiffness were provided. The longitudinal and transverse three-point bending tests of the hat-stiffened laminates were designed to measure the mid-span deflection of the beam and the obtained test data were compared with the analytical solution corresponding to the equivalent bending stiffness. The deflection at the center point of longitudinally and transversely hat-stiffened composite beam in three-point bending and the four-sided simply-supported hat-stiffened composite laminates was calculated by ABAQUS and compared with the analytical solution corresponding to the equivalent bending stiffness. The results show that the analytical solutions are in good agreement with the numerical calculations and the experimental results, which verifies the correctness of the analytical expression of the equivalent bending stiffness. It can be used confidently in the design of the hat-stiffened panel.