Abstract: To reduce the thermal deformation, the carbon fiber reinforced polymer(CFRP) circular cell honeycomb core was proposed and designed. The external shear modulus of the core, which plays an important role in calculating the thermal deformation of the sandwich structure based on Reissner theory or ABAQUS software, was discussed in present study. The analytical result of external shear modulus was given based on energy equivalent theory from the stress and strain. Taking the usual lay-up made of T300/epoxy for example, the finite element solution of core calculated by ABAQUS software was compared with the theoretical one, which demonstrates that the maximum error is 10.4%. Using core with (±45°)₂ layup and made of T300/epoxy, the double-shear test was carried out. It proves that the error between the experimental and the theoretical solution is 24.1%, which is close to the error of honeycomb core in previous study. The final finite element analysis shows that the 24.1% error has less effect on the mechanical characteristics of whole sandwich structure. Accordingly, analytical expressions for external shear modulus are of great value for engineering application. All the analysis can help for the design of the CFRP circular cell honeycomb core in the future.