The invention discloses a preparation method for an ammonium polyphosphate/kaolin composite flame retardant. The preparation method specifically comprises the following process steps: (1) calcining superfine water-washed kaolin for 1 to 2 hours at 600 to 900 DEG C to obtain calcined kaolin; (2) performing surface modification on ammonium polyphosphate by using cyanurtriamide to obtained modified ammonium polyphosphate, wherein the modifying temperature is 100 to 250 DEG C, the modifying time is 1 to 2 hours, and the using amount of the cyanurtriamide is 5 to 25 percent of the mass of the ammonium polyphosphate; (3) mixing the calcined kaolin and the modified ammonium polyphosphate in a mass ratio of 1:2-6, and performing compounded surface modification by using a modifier to obtain the ammonium polyphosphate/kaolin composite flame retardant, wherein the modifying temperature is 60 to 120 DEG C, the modifying time is 15 to 60 minutes, and the using amount of the modifier is 0.5 to 3 percent of the total mass of the calcined kaolin and the modified ammonium polyphosphate. According to the preparation method for the ammonium polyphosphate/kaolin composite flame retardant, the water solubility and the hygroscopicity of the ammonium polyphosphate can be reduced obviously; the dialysis resistance of an ammonium polyphosphate filled polymer product can be improved.