Abstract: To accurately solve the buckling stability of underwater composite pressure-bearing cylindrical shells, hydrostatic pressure was decomposed into triaxial prestresses instead of the traditional membrane stresses. Based on three-dimensional elasticity theory and state-space techniques, a state-space equation was established with displacement and transverse stresses as state vectors. The differential quadrature method was used to establish the discrete form of the state-space equation for non-simply supported cylindrical shells. In this way, the analytical solutions of the buckling problems of simply-supported and clamped composite cylindrical shells were achieved. By comparing with the existing theoretical literature, experimental results and finite element methods, the correctness of the theoretical model and solution strategy has been verified. The study shows that the influence of boundary conditions on the shell stiffness gradually weakens with the increase of the length-to-diameter ratio. The critical buckling loads of cylindrical shells under different boundary conditions tend to be close as the length-to-diameter ratio increases. For simply-supported cylindrical shells, the critical buckling load first increases and then decreases with the increase of the lay-up angle, reaching the maximum value at 45°. For clamped cylindrical shells, the critical buckling load gradually decreases as the lay-up angle increases.