The invention discloses a preparation method for nano-silicon dioxide-modified ammonium polyphosphate. The preparation method comprises the following steps: (1) adding reactants, namely, phosphorus pentoxide and diammonium phosphate, in an equal molar ratio in a kneading machine, stirring uniformly, heating, intruducing ammonia, reacting for 0.5 to 5 hours at 180 to 320 DEG C, adding nano-silicon dioxide, continuing introducing the ammonia, and reacting for 0.5 to 5 hours at 180 to 320 DEG C to obtain a coarse product of the nano-silicon dioxide-modified ammonium polyphosphate; (2) adding the coarse product of the nano-silicon dioxide-modified ammonium polyphosphate into the kneading machine, spraying alkyl siloxane with stirring, and reacting for 0.5 to 5 hours at 80 to 180 DEG C to obtain the nano-silicon dioxide-modified ammonium polyphosphate. The nano-silicon dioxide-modified ammonium polyphosphate is white powder. The preparation method is simple in process steps, high in operability and low in preparation cost; the prepared nano-silicon dioxide-modified ammonium polyphosphate is good in flame retardance and compatibility, and has migration resistance.