A preparation method for a water-insoluble crystal form ammonium polyphosphate includes conveying crystal H-type phosphorus pentoxide and diammonium phosphate at a molar ratio of 1:1-1.5 into a malaxator with twin screws, and injecting ammonia gas for 3-40 minutes into the malaxator at 20-25 m3/h while preheating the material at a temperature between 100° C. and 300° C. Therafter one injects ammonia gas again at 8-15 m3/h for 4-8 hours. The resulting material is transferred to another sealed container and stirred while coupling agent is added. Stirring continues until the material drops below 60° C., the material then being transferred into organic solvent for washing for 30-50 minutes. Thereafter the material is first filter pressed, then subjected to low temperature drying, and then sieved to obtain the final product.