Abstract: Panel separating from honeycomb core caused by the adhesive property decreasing is the main reason which make the mechanical properties of honeycomb sandwich panel degenerate at high temperatures. Equivalent debonding coefficient and temperature retention coefficients of elastic modulus of panel and honeycomb core were defined as damage variables to reflect the extent of the damage for the bending stiffness of honeycomb sandwich panel at high temperatures. A mechanics model for bending stiffness of honeycomb sandwich structure decreasing with temperature rising was created. It shows that the physical model can reflect the principle of the bending stiffness degenerating with temperature rising perfectly. The difference between the calculated and the experimental result is less than 15%, which achieves the goal that forecasting the bending stiffness of honeycomb sandwich panel at high temperatures correctly. This research can be used to estimate the bending stiffness of soft honeycomb sandwich structure at high temperatures.