The fiber reinforced geopolymer was prepared from the main raw materials of fly ash and steel slag. The mechanical properties, mass loss rates and surface of fiber reinforced geopolymer were explored under high temperature, and the improvement mechanism of high temperature resistance of geopolymer by fibers was analyzed. The results show that fibers improve high temperature resistance of geopolymer. From 25℃ to 300℃, the mechanical performance of samples rise with increment of temperature; but it begins to decline when the temperature rises to 300℃; and decreases significantly at 700℃. When the temperature reaches 900℃, except for sample SF 4, the strength of the other samples increased slightly. The mass loss rate of each samples is about 10% at 900℃, but all samples remain intact without obvious deformation. The correlation coefficient R2 of the curve between geopolymer strength and temperature has reached about 09, indicating that the fitting result is accurate. The thermodynamic mathematical model between strength and temperature has also been proposed.