Accelerated hydrothermal aging test and fire test of 105 intumescent coating protected steel specimens were conducted to investigate the effect of two types of topcoat on anti aging and insulation properties of intumescent coating for steel elements. The results show that compared to specimens without aging, the constant effective thermal conductivities of specimens after 84 cycles of hydrothermal aging test but without topcoat increases by 131%. Applying topcoat reduces the effect of aging and the constant effective thermal conductivities of specimens after 84 cycles but with 2 layers of type E topcoat and two layers of type A topcoat increases by 42% and 21% respectively. Applying topcoat can also limit expansion of intumescent coating. When intumescent coating experiences prolonged cycles of aging test, the protective effect of topcoat prevails and using topcoat can increase insulation properties of intumescent coating. Otherwise, using topcoat will result in worse properties of intumescent coatings.