Based on a simplified mathematical model involving certain approximations, a simple equipment was designed for testing the fire resistant ability of the fire resistant coating. Six key variables were involved in the model. Numerical design of experiments allows identifying the influent factors which govern the intumescent behavior. It was found that i, a parameter reflecting the comprehensive effect of heat insulation ability of exterior carbonization layer as well as the heat source, shows the most important effect, and its formula is obtained by fitting of the curve. In the end, this method was carried out on a series of samples of a fire resistant coating with different original thickness, and its i value was calculated.