The insulative properties of intumescent coatings will degrade after aging and the degradation process is affected by such factors as coating type, environmental condition, coating thickness and service time. In order to investigate how insulative properties of the coatings are affected by those factors, accelerated aging tests and fire tests were carried out on 78 steel specimens coated with intumescent coating. Test results show that type A coating is vulnerable to hydrothermal environment; thicker coatings delay degradation process; effective thermal conductivities of intumescent coatings increase with prolonged service time and the relationship of effective thermal conductivity service time is affected by coating type and environmental condition. Physical and chemical equilibrium of intumescent coating system is reached in condition No. 1 after 10 years (21 cycles). Effective thermal conductivities of intumescent coating increase progressively in condition No. 2 and condition No.3.