The invention belongs to the technical field of preparation of bismuth ferrite and discloses a method for preparing pure-phase nanocrystalline bismuth ferrite. The method comprises the steps of carrying out high-energy ball milling on ferric oxide and bismuth oxide, which serve as raw materials, through mechanical alloying so as to obtain amorphous bismuth ferrite, and then, separating out a purebismuth ferrite nanocrystal structure from the amorphous bismuth ferrite through a high-temperature heat treatment method. The bismuth ferrite prepared by the method is of the pure bismuth ferrite nanocrystal structure.