Silicon nitride was prepared by means of low-pressure preheating combustion synthesis in this paper. When N2 pressure is less than or equal to 2MPa, the lowest preheating temperature is about 653K, reactant could make silicon nitride by combustion synthesis. The additive of ammonium fluoride and ammonium chloride, as the reactive diluent, and Si react and make some useful intermediate products to α -Si3N4 after thermal decomposition. The reactive diluent content must be appropriate. At the same time, with the improvement of Si3N4, as the inert diluent, content in reactant, the adiabatic flame temperature of system would reduce, and combustion reaction rate of Si also drops, so it is propitious to make high level of α -Si3N4.