The state and formation mechanism of α-Si3N4 in Fe–Si3N4 prepared by flash combustion were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that α-Si3N4 crystals exist only in the Fe–Si3N4 dense areas. When FeS i75 particles react with N2, which generates substantial heat, a large number of Si solid particles evaporate. The product between Si gas and N2 is a mixture of α-Si3N4 and β-Si3N4. At the later stage of the flash combustion process, α-Si3N4 crystals dissolve and reprecipitate as β-Si3N4 and the β-Si3N4 crystals grow outward from the dense areas in the product pool. As the temperature decreases, the α-Si3N4 crystals cool before transforming into β-Si3N4 crystals in the dense areas of Fe–Si3N4. The phase composition of flash-combustion-synthesized Fe–Si3N4 is controllable through manipulation of the gas-phase reaction in the early stage and the α→β transformation in the later stage. The state and formation mechanism of α-Si3N4 in Fe-Si3N4 prepared by flash combustion were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that α-Si3N4 crystals exist only in the Fe-Si3N4 dense When FeS i75 particles react with N2, which generates substantially heat, a large number of Si solid particles evaporate. The product between Si gas and N2 is a mixture of α-Si3N4 and β-Si3N4. At the later stage of the flash combustion process, α-Si3N4 crystals dissolve and reprecipitate as β-Si3N4 and the β-Si3N4 crystals grow outward from the dense areas in the product pool. As the temperature decreases, the α-Si3N4 crystals cool before transforming into β-Si3N4 crystals in the dense areas of Fe-Si3N4. The phase composition of flash-combustion-synthesized Fe-Si3N4 is controllable through manipulation of the gas-phase reaction in the early stage and the α → β transformation in the later stage.