The effects of film-forming substances epoxy emulsion, acrylic emulsion and styrene-acrylic emulsion on the fire resistance of water-based ultra-thin fireproof coatings were investigated by using the expansion multiplier and back temperature of steel plates as indicators. The results show that the highest expansion multiplier of 14.4 and the lowest back temperature of 284.3 ℃ for 80 min were obtained for the composite coating with epoxy emulsion∶acrylic emulsion=1∶1. The softening temperature of the film-forming substance matched with the decomposition temperature of the expansion (P-C-N) system, which could effectively wrap the gas generated by the P-C-N system, promote the formation of porous structure of the carbon layer and block the heat transfer. The decomposition of organic substances such as film-forming substances is complete in combustion, and the surface of porous carbon layer contains TiO2 and TiP2O7 inorganic components, which further enhance its heat shielding and anti-oxidation properties, with a final residual weight of 28.18% and good thermal stability.