Abstract: Based on the wastes of fly ash (FA) and chitosan (CS), FA-CS composite flame-retardant filler was prepared and introduced into the water-based intumescent fireproof coating system to strengthen the fire-resistant limit and heat-insulating property of the coating. The FA-CS composite was observed and characterized by the techniques of FT-IR, XRD, SEM, etc. Then the fire resistance and heat insulation ability of the coating were evaluated by large-plate experiments, back-temperature curves and microstructure and thus revealed its fire-retardant and flame-retardant mechanism. Large-plate experiments showed that on the premise that the external flame temperature was maintained at 1000±50℃ and the combustion time is 60 min, compared with the Waterborne intumescent system(WIS), FA/WIS, and CS/WIS coatings, the FA-CS/WIS coating exhibited a higher expansion multiplier (10.2 times) and lower backside temperature (279℃), thus demonstrating excellent ability to block the heat transfer and flame washout. In addition, SEM analysis of the surface morphology of the intumescent layer of the coating showed that the FA-CS/WIS coating had a denser and smoother surface compared with the other coatings. In brief, The excellent flame retardant and fire-resistant properties of the FA-CS/WIS coating were mainly attributed to (a) the acidification and gasification of the intumescent flame retardant system of the coating material, which gradually formed the intumescent layer; (b) the participation of chitosan in the charcoal-forming reaction of the intumescent layer, which promoted the intumescent multiplication; and (c) Fly ash with excellent heat-resistant properties in the residual charcoal after combustion is used as a temperature-resistant material to fill in the pores of the expansion layer, which increases the thermal stability and barrier capacity of the expansion layer.