Abstract: The instability of ring-stiffened cylindrical shell, based on the Reissner-Naghdi theory of shell and the Timoshenko theory of ring beam, is investigated using Ritz method. The effects of shear deformation of both shell and ring on critical loads are considered for simple supported boundary condition. As the ring restricts the deformation of shell, the pre-buckling state of cylindrical shell subjected to uniform external pressure should be bending, and the circumferential compress resultants should be nonuniform distributing. The influences of pre-buckling state both with bending and without bending on critical loads are analyzed. Numerical results of critical loads for computational examples have good agreement with the experimental values in reference. The results show that the effects of shear deformation on instability of ring-stiffened cylindrical shell are considerable in some situations.