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Development of effective methods for quantificationally characterizing the thermal resistance of the intumescent char layer of fired fire-retardant coating is of interest in many academic researches and industrial applications. In this work, a novel approach is proposed to calculate the thermal resistance of the intumescent char layer produced after the ultra-thin fire-retardant coating exposed to fire. A correlation was established based on the relative thermal resistance of the char layer to that of steel plate and the surface temperature at the back of the coated steel plate under steady-state heat transfer. The thermal resistance of the steel plate was obtained about 0.009 K·m2·W−1 by calculating the heat flux based on the radiation heat exchange involving three-surface enclosure model (flame, steel, and wall). The thermal resistances of the char layers without and with the layered double hydroxide (LDH) were 0.011 K·m2·W−1 and 0.013 K·m2·W−1, respectively. The results show that the external roughness and internal honeycomb structure can increase both the conductivity and the convective thermal resistances of the char layer to protect the steel structures. The porosity of the char layer is between 0.15 and 0.18, and its thermal conductivity is between 0.4 and 0.8 W·m−1·K−1 in the process of fire. Unlabelled Image • A novel method is proposed to calculate thermal resistance of intumescent char layer. • Thermal resistance is used to evaluate heat-shielding performance of char layer. • LDH improves external roughness and internal honeycomb structure of char layer. • Porosity of char layers is obtained to be 0.15–0.18 via SEM images processing. • Thermal conductivity of char layers is 0.4–0.8 W·m−1·K−1 in the process fire. [ABSTRACT FROM AUTHOR]