In order to investigate the aging mechanism and degradation process of intumescnet coating, the results of fire tests are reported. FTIR tests and SEM tests were conducted on the intumescent coating after hydrothermal aging tests. Results of the degradation mechanism study reveal that the hydrophilic components in intumescent coating move to the surface of the coating and can be dissolved by moisture in the air. Compared to specimens without hydrothermal aging tests, after 42 cycles of hydrothermal aging(to simulate 20 years of exposure to an assumed exposure environment), the effective thermal conductivity of typeU intumescent coating is 50% higher and that of typeA intumescent coating is 100% higher than fresh coating.