The flexural properties of Nomex honeycomb sandwich structure at different temperatures were studied with three-point bending method. Considering the honeycomb′s orthotropy, L and W specimens were made, respectively taking the two main directions in the core plane as the longitudinal direction. Bending experiments were carried out to determine the bending stiffness and shear stiffness at 25 ℃, 50 ℃, 100 ℃, 150 ℃ and 180 ℃ respectively. The results show that both the bending stiffness and the shear stiffness in the two directions decreases as the temperature rises. When the temperature rises to 180 ℃, the two kinds of stiffness are less than 20% of that at room temperature. The temperature also has effect on the failure mode when the sandwich structure is under transverse mechanical load. A sunken skin is often seen when the temperature is below 50 ℃; when the temperature rises to 150 ℃ or higher, the skin may be separated from the honeycomb and be protruded outward. The research results may provide a reference for the application of Nomex honeycomb structure in laser irradiation or fluctuating temperature environments.