The invention discloses a preparation method of a nano iron fluoride-based composite material, which aims to solve the problems of high raw material cost, complex synthetic process, difficulty in uniform mixing of iron fluoride and carbon and oxygen elements, and inability for effective grain size control of the conventional preparation method. The preparation method of nanocrystalline iron fluoride comprises the following steps: (1) preparing a solution consisting of an iron source precursor, a polyalcohol solvent, and a complexing agent, and carrying out refluxing at a temperature of 30-80 DEG C; (2) adding a fluorinating reagent to a solution obtained through (1) for fluorination, and carrying out refluxing and stirring at a temperature of 140-200 DEG C to obtain a suspension; and (3) carrying out aging of the suspension at a temperature of 100-160 DEGC, carrying out washing and filtering of the aged suspension to obtain a solid, and finally calcinating the solid at a temperature of 200-350 DEG C to obtain the nano iron fluoride.