A new numerical method allowing one to estimate a temperature field in an intumescent fire-retardant coating is proposed. The data on heat conductivity of the material, on kinetics of its decomposition, and on kinetics of change of its rheological properties serve for calculation of input parameters. On the basis of experimental information about curing kinetics, the algorithm computes a viscosity of the material in each elementary layer of the coating. It is assumed that the local change of viscosity, in turn, predetermines a local expansion coefficient of the coating. The results of the calculations are compared with the experimental data obtained for the coating on the basis of phenol-formaldehyde resin and boron oxide.