Abstract: If the standard finite element method is used to analyze stability of stiffened circularcylindrical shells under external pressures, one must adopt nodal displacements determinedof elements to describe variation of various buckling wave shapes undetermined, which certainly increases the element number, complicacy of base functions, and needs a major inner memory capacity of the computer, so that it is very difficult to analyte stability.In this paper, the semi-analytical finite element method is used to analyze stability of cylindrical shells, thus analytical achievements are substituted for discretization of the standard finite element method, therefore the difficult problem encountered by the standardfinite element method is solved effectively. Characteristics of the semi-analytical finiteelement method are that in the displacement models, analytical functions of the cylindricalshell and the round ring plate are introduced in the circumferential direction, and discrete interpolating polynomial functions are adopted in the axial and radial directions. Because the combined displacement models of the analytical and discrete functions are adopted, the freedom number is reduced, calculating accuracy is improved, and various complicated structures as well as their different joint forms can be treated conveniently.In the paper, we adopt the geometrical nonlinear large-deflection theory, and derive the elastic and geometrical stiffness matrixes of the element from it. In stability calculation,solution of the buckling load and the buckling deformation shape is converted into so iution of the maximum characteristic root and its characteristic vector of the real matrix on math. Finally, some calculating examples are given, and the calculating results are better agreement with the analytical solution, which demonstrates the method in the paper is practical and able to find wide use.