The invention relates to a method for preparing diphosphonate melamine salt (pentaerythritol phosphate), which includes the following steps: firstly, pentaerythritol and phosphorus pentoxide are mixed in an extruder to carry out esterification reaction and generate phosphate acid (pentaerythritol phosphate); then, the diphosphonate (pentaerythritol phosphate) and melamine or the derivatives thereof are mixed to carry out ammonium salinization reaction under the existence of a solvent, and finally moisture is removed to obtain the diphosphonate melamine salt (pentaerythritol phosphate). The method uses the phosphorous pentoxide as a raw material which does not generate hcl waste gas and does not need to recycle the trichloro-phosphate. Meanwhile, the method can carry out the esterification reaction without using an organic solvent, which can shorten the required time of the esterification reaction and avoid the danger of the explosion generated by high pressure in a closed reaction system.