The invention discloses a method for preparing nano-crystal magnesium fluoride with a high specific surface area, which aims at solving the problems that magnesium fluoride preparation has small high-temperature specific surface area and poor stability and an existing preparation method is expensive and complicated and has heavy pollution. The method comprises the following steps: (1) preparing a solution containing magnesium-source precursor, polyhydric alcohol and coagulation accelerator, and refluxing at 20-80 DEG C; (2) fluoridizing the solution obtained in the step (1) with a fluoridizing reagent, and aging at 100-160 DEG C to obtain liquid sol; and (3) drying the liquid sol at 140-200 DEG C to obtain solid gel, and roasting at 350-450 DEG C to obtain nano-magnesium fluoride.