The work aims to prepare a kind of fire barrier bonded with C-S-H gel and rich in ettringite content with metallurgical slag and sulfoaluminate cement, in order to meet the higher fire protection demand of underground pipe gallery. The performance of sample at high temperature was investigated via calcination test and thermal analysis combined with means of phase determination, microstructure observation and pore structure examination. Adding sulfoaluminate cement to metallurgical slag system was conducive to the formation of ettringite during hydration. The heat absorption of sample increased from 73.27 J/g to above 109.40 J/g at dehydration temperature range of 50 ℃ to 350 ℃ and the compressive strength also increased from 37.18 MPa to above 40.68 MPa. The results of spray fire testing at 1 000 ℃ for 30 min showed that the backfire temperature of the sample prepared at a thickness of 10 mm lied between 383 ℃ and 336 ℃, which decreased along with the increasing content of sulfoaluminate cement and the sample had superior property in fire protection than the glass fiber - magnesium oxychloride (GFMO) board and calcium silicate autoclaved (CSA) board. The thermal protection mechanism of the sample is due to the combined effect of heat absorption in dehydration of crystallization water-containing phases, thermal insulation of porous layer derived from dehydration, and maintenance of macrostructural integrity for sample at high temperature.