Abstract: In view of the problems of hydration cracking and powdering, uncontrollable gelation time and poor compressive strength of sodium silicate gel, poor fluidity, small penetration range and high cost of organic gel, combined with the characteristics of strong plasticity and non-cracking of bentonite and fly ash gel, by introducing fly ash and sodium silicate gel, the composite gel was prepared by one step mixing method with sodium bentonite as the main raw material. The optimum ratio of the composite gel was studied by gelation time, compressive strength and viscosity test. The gelation time of the composite gel is controllable between 30 s and 15 min, and the solid/water ratio is suitable from 1∶8 to 1∶16, which has the characteristics of wide solid/water ratio, and the maximum compressive strength is 25 MPa. On this basis, the surface morphology of composite gel fire prevention materials was characterized by scanning electron microscopy, and the microstructure of gel was analyzed. The results showed that Al-O-Si three-dimensional network structure was crosslinked with calcium silicate hydrate（Ｃ－Ｓ－Ｈ） and aluminum silicate hydrate（Ａ－Ｓ－Ｈ） in the microstructure of the composite gel. Based on this, the gelling mechanism of the composite gel was proposed.