The invention discloses a preparation method for an ABC dry powder extinguishing agent. The method comprises the steps that monoammonium phosphate, ammonium sulfate, nanosilicon dioxide, mica, melamine, pentaerythritol, a manganese compound, borax, copper acetate and a coupling agent are mixed, silicone oil is injected into the mixture at the temperature ranging from 50 DEG C to 60 DEG C to obtain a product D, the product D is smashed through a continuous modification mulch applicator until the diameter of more than 50% of particles is smaller than 40 micrometers, stirring is carried out at high speed at the temperature being 70 DEG C for 30 min to 40 min, drying is carried out at the temperature ranging from 90 DEG C to 100 DEG C for 2h to 3h to obtain a product, the temperature is decreased to be 60 DEG C, cooling is carried out for 10-20 min, and then the ABC dry powder extinguishing agent is obtained. The obtained ABC dry powder extinguishing agent is high in efficiency and small in smog release amount during extinguishment.