Steel structures are widely used in the field of engineering building because of its high toughness, high strength, easy processing and other advantages. However, steel structures will lose their mechanical properties under high-temperature environments, causing structural failure. Therefore, fire protection becomes crucial to the safety of steel structures. In this study, using metakaolin, slag and hydrophobic-treated expanded perlite as main raw materials, and potassium water glass with modulus of 1.5 as activator, a non-expanded potassium-based a geopolymer fire resistance coating was prepared. The fire resistance performance of coating was characterized by the big panel method under 1 200 ℃. At the same time, the mechanical properties, apparent morphology, phase composition, and microstructure evolution of the geopolymers before and after heat treatment at room temperature, 1 000 ℃ and 1 100 ℃ were characterized and analyzed, and the ceramicization process of geopolymers at high temperature was explored. The results show that the geopolymer coating has great fire resistance properties. The temperature on back of the steel plate is lower than 160 ℃ after exposure to 1 200 ℃ for 2 hours. The compressive strength of coating increases to 30.80 MPa after heat treatment at 1 100 ℃ for 2 h, which is 5.8 times of that under room temperature. The amorphous geopolymer phases of fire retardant coating starts ceramization at 800 ℃ and the main crystalline phases after ceramization at 1 100 ℃ are anorthite, leucite and mullite.