Abstract: In order to solve the problem of deep-hole sluice gate in the main beam of previous slender beam theory which the calculation is unsafe and the finite element (FE) method is computationally expensive and conceptually complex, a new concise and accurate analytic approach is desirable for this specific cross-sectional design. Based on the theory of elasticity, formulas for the calculation of longitudinal compressive stress of an Ⅰ-beam with a uniformly distributed load as well as for the calculation of bending normal stress, while taking into account the longitudinal fiber extrusion and shear warping, were derived. Furthermore, determination criteria for composite thin-walled deep beams were proposed and various analytical methods for bending normal stress calculation were evaluated based on the FE numerical simulation results. The results demonstrate that the existing analytical methods produce fairly large errors and cannot accurately reveal the pattern of being normal stress variation of composite thin-walled deep beams, whereas the method developed in this research, by considering sheer warping and interlayer extrusion under biaxial stress state, is able to generate more accurate calculation results with an extended scope of application.