The subject of the study was thin-walled composite structures, which were made of carbon-epoxy composite. The structures were characterized by a closed square cross-sectional shape. In the research paper, stability tests of axially compressed thin-walled composite structures were carried out in order to determine the critical state of the structures. Experimental tests were carried out on a universal testing machine, using a system for optical measurement of deformation of structures. Numerical simulations were carried out using the finite element method. The research made it possible to evaluate the work of the structure in the buckling state from a qualitative and quantitative point of view.