In order to obtain the stress states of flat extrusion cylinder structure under various working conditions and verify whether the cylinder structure meets the working strength requirements under high temperature and high pressure conditions, the load characteristics for each work stage of flat extrusion cylinder from assembly to pressurization were considered comprehensively, and according to the strength calculation method of thick-walled cylinder structure based on elastic mechanics theory, the stress calculation formula for each layer structure of extrusion cylinder was derived. Then, the key factors affecting the strength of flat extrusion cylinder were summarized as the ratio of inner to outer diameters, working pressure, interference fit coefficient, Young′s modulus of materials and temperature, and the treatment methods of these key factors in numerical calculation were analyzed. Furthermore, the thermal and structural coupling analysis on the flat extrusion cylinder was carried out by finite element method, and the deformation and stress states of the loaded structure of flat extrusion cylinder under five different conditions were simulated to obtain the safety factor of each layer for the structure, which intuitively displayed the safety margin of each part for the structure and provided the theoretical support for manufacturing and optimization design of flat extrusion cylinder.