Anti thermoaging property of water borne ultra thin intumescent fire retardant coating for steel structure with epoxy resin as binder was studied by means of FTIR, TGA, SEM, etc. The results show that the fireproof property, thermal stability, anti ablation under high temperature and char strength of the coating decrease with the increase of the aging temperature. FTIR results indicate that this was because of the migration behavior of the fire retardant additives during the aging process. The fire retardant additives were lessened due to this migration behavior, and the loss of the additives increases with the rise of the aging temperature. Further study proved that the loss of the fire retardant additives changed the intumescent behavior. The cell size in the foam structure increases, and their distribution becomes non uniform; this also reduces the strength of the char layer.