The theory of thin-walled bars is important because light steel thin-walled structures are widely used. Traditionally, in calculations two theories are used: theory for open-profile and closed profile bars. The calculations are difficult, because different finite elements are used for different bar types. In 2005 V.I. Slivker worked out a semi-shear theory, which is suitable for thin-walled bars of open sections and closed sections. Similarly, this article presents the research on finite element modeling for the stability problems of thin-walled bars using the same theory to the geometric stiffness matrix. It was shown that the FEM solution converges to the exact one as the number of the finite elements increases. The numeral solutions were compared to critical forces obtained by the classical Euler formula. It was found that using the cross-sections as the thin-walled ones can reduce the critical force, especially for the open cross-sections.