<p>The application scope of steel structures is extensive. In order to improve the fire resistance of the steel structures, fire-retardant coating can be brushed on the surface of the steel structures. In this study, a thin intumescent fire-retardant coating (thin-IFRC) for steel structures is chosen to conduct a series of experiments including thermogravimetric and Fourier transform infrared spectrometry experiments. The results showed that the whole pyrolysis process of thin-IFRC could be divided into two stages: Stage I, the decomposition of melamine and a small amount of ammonium polyphosphate to produce NH₃ and CO₂ with the mass loss being 10.44%. Stage II, the decomposition of large amounts of ammonium polyphosphate and pentaerythritol to produce CO₂ and H₂O with the mass loss being 36.65%. The activation energy <i>E</i>_<i>a</i> and reaction mechanism of thin fireproof coating were also calculated by coupling the model-free and model-fitting methods. The reaction mechanism of the thin fireproof coating was determined and verified by the kinetic compensation effect. Eventually, the reaction pathways of thin-IFRC pyrolysis were put forward. The obtained pyrolysis characteristics would be of great reference value for fire protection of steel structures.</p>