The thermal radiation intensity on the XPS board, the ignition time, the mass loss rate and the surface temperature were measured in 10 sets of ignition experiments of 2.2cm and 4.5cm thick XPS boards. The ignition characteristics of XPS boards in different linear heat flux were studied. The experiment results show that the 0.0194kW/（m2·s） heat flux is not high enough to ignite the samples. Under the heat flux exceeding 0.0373kW/（m2·s）, XPS board was heated to the critical ignition temperature and then the sample was suddenly burnt with the glaring flame and black smoke. After combustion of XPS board, the temperature decreased rapidly from 970.5℃ of maximum temperature. And the experiments also show that the mass of XPS boards decreases linearly with time during the rapid burning stage, the critical ignition temperature is 346.0398.0℃, the ignition time has power relation to the heat flux rate with power exponent of -0.765.