Sodium chloride is used as the matrix to prepare superfine dry powder fire extinguishing agent for metal fire. The anti-solvent method was used to modify it, so that the nano-hydrophobic silica was adsorbed on the surface of sodium chloride crystal, and the dispersant was PEG-1000. The results show that the addition of nano-hydrophobic silica is beneficial to the refinement of sodium chloride particles, but excessive addition will have an adverse effect. The optimal dosage is about 3 wt%, and the average particle size is 2.18 μm.