In order to improve fire extinguishing properties and thermal stability of gel, the gelation characteristics of ammonia-free compound gel were investigated by photoelectric analysis. The gelling time of ammonia-free compound gel was measured by photoelectric method and the effect of temperature on gelling time of compound gel was analyzed and the thermal stability of plain gel and compound gel were compared with each other in different environments. The results show that the gelling time of compound gel shortens rapidly with temperature increasing and the influence of environmental temperature on the gelling time has a relationship with the mass fraction of compound gel, the degree of influence is that 6% gelatinizing agent＞8% gelatinizing agent＞10% gelatinizing agent. Adding gelling agent composed of varied complex Lewis acid to Na₂SiO₃·nH₂O solution could counteract the charge of the micelle silicate, damage the fusion membrane of the micelle silicate, and promote colloidal particle coagulation in order to convert into full of flexibility. Because the composite gelling agent is crushing polyhedron, the increases of specific surface area and water absorption improve the thermal stability of the gel.