The invention relates to a preparation method of high-polymerization-degree melamine polyphosphate. The preparation method includes the following steps that firstly, melamine is completely dissolved in deionized water, phosphoric acid is dropwise added, a modifier is dropwise added into the solution after a reaction lasts for a period of time, then, the reaction continues for a period of time, finally, the reaction product is cooled, filtered, dried and smashed, and a modified intermediate is obtained; secondly, the modified intermediate is heated to 200 DEG C first under atmosphere protection and/or the vacuum condition and then is heated to 380 DEG C at the rate of 0.1-0.5 DEG C/min, a thermal condensation reaction is conducted, and high-polymerization-degree melamine polyphosphate is obtained, wherein the polymerization degree n ranges from 170 to 190, and the temperature for 1% weight loss of high-polymerization-degree melamine polyphosphate is higher than 375 DEG C. By means of the preparation method, the problems that melamine polyphosphate in the prior art is low in heat stability, low in flame retarding efficiency, poor in compatibility and the like are solved, and the processing requirement for flame-retarding forming of glass fiber reinforced nylon and polyester can be met.