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 FeSi75 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.