Abstract: An equivalent stiffness calculation method based on refined analytical model of composite laminated box beam was proposed. Considering the three-dimensional strain effect, the two-dimensional reduced modulus component of the composite single layer was used to represent the three-dimensional reduced modulus component, which simplified the calculation of the equivalent stiffness coefficient of the composite laminated box beam, and the analytic formulas of equivalent bending stiffness and equivalent torsional stiffness characterized by the cross-section geometry of the beam and the stiffness coefficient of the laminate were obtained. The analytical formulas are suitable for composite laminated box beam with circumferentially uniform stiffness, and fully consider the effects of bending-shear coupling and torsion-tensile coupling on equivalent stiffness. Through the three-point bending test and the torsion test, the correctness of the analytical formula is verified. By comparing with the layered equivalent superposition method and the finite element method, the analytical precision of the analytical formula is analyzed. Combined with the classical laminated plate theory, the influence of the lamination method on the equivalent stiffness and its causes were studied, and the variation law of the equivalent stiffness of the laminated composite beams with different lay-ups was predicted.