A series of fire tests were carried out on intumescent coating protected steel plates both with and without top coating. The expansion ratio and effective thermal conductivity of intumescent coatings with and without top coating were compared. The results show that top coating reduces the expansion ratio of intumescent coating by around 30% of the final thickness. The effective thermal conductivity is obtained as the average thermal conductivity of intumescent coating within the steel temperature range of 400600℃, and can be used to present the insulating properties of intumescent coating with top coating. The proposed effective thermal conductivity can be expressed in linear relationship with the dry film thickness(DFT) of intumescent coatings with and without top coating. The effective thermal conductivity of intumescent coatings with top coating is larger than that without top coating by 30%70% under the same DFT.