［Objective］To enhance the fire resistance and salt spray resistance of intumescentfire-retardant coatings for steel structures using expandable graphite（EG）as a synergist.［Methods］ The effects and mechanisms of EG on the fire resistance and salt spray resistance were investigatedusing cone calorimetry，thermogravimetric analysis（TGA）， and neutral salt spray testing.［Results］ The addition of EG could effectively enhance the coating′s fire resistance and smoke suppressionproperties. The peak heat release rate and the peak smoke generation rate of the coating after the residual carbon rate at 700 ℃ was increased to 28. 3%. This enhancement was primarily attributed tothe formation of worm-like structures by EG during the combustion process，which reinforced the denseness of the expanded carbon layer to block the heat and material transfer more effectively. Saltspray tests revealed that exposure to a neutral salt spray environment could damage the structuralintegrity of the coating，leading to an increase in the heat release rate and smoke production. However， the addition of EG could delay the coating′s aging process induced by salt spray to a certain extent.Consequently，even after aging，EG-modified coatings maintained lower heat release，reduced smoke production，and superior carbon layer stability compared to unmodified coatings. Notably，the residual carbon rate at 700 ℃ for the aged EG-modified coating was 15. 4% higher than that of the aged coatingwithout EG.［Conclusion］ EG as a synergist of intumescent steel structure fire-retardant coatingscould significantly improve the fire resistance and salt spray resistance of the coating.