With the continuous expansion of silicon nitride (Si3N4) ceramics in application fields such as aerospace, electronic devices, and high-temperature structural materials, the demand for high-quality Si3N4 powder has been increasingly growing. However, the traditional pyrolysis of silicon imine [Si(NH)2] usually faces the problems of high crystallization temperature and whisker formation, which affects the quality and application performance of the final Si3N4 powder. The innovation of this study is to use molten salt environment to reduce the crystallization and phase transition temperature of Si(NH)2, control particle size and inhibit the formation of irregular whiskers. In this study, α-Si3N4 powder with regular morphology is prepared by molten salt pyrolysis method, using Si(NH)2 as raw material and sodium chloride (NaCl) as reactive medium. Meanwhile, the effects of pyrolysis temperature and the ratio of silicon to salt on the phase composition, particle size distribution, and microstructure of the prepared powder are investigated. The results indicate that NaCl molten salt can significantly lower the crystallization and phase transition temperatures of Si(NH)2. Under the condition of holding at 1400℃ for 2 h, Si(NH)2 is completely converted into Si3N4 powder with high crystallinity and regular morphology. The content of α-Si3N4 is 75.52wt%, and the average particle size is 905.40 nm. In addition, with the increase of NaCl content, the α-Si3N4 content in the powder first decreases and then increases, whereas the powder particle size gradually reduces. This method effectively inhibits the formation of whiskers during the pyrolysis of Si(NH)2 and lowers the crystallization temperature of α-Si3N4 powder. Mechanistic analysis reveals that in the liquid-phase environment provided by the molten salt, Si(NH)2 promotes the growth of α-Si3N4 particles through a dissolution-precipitation mechanism. Overall, This method provides a new strategy for preparing high-quality Si3N4 powder with regular morphology and exhibits broad application potential.