The invention belongs to the technical field of yttrium doped bismuth ferrite preparation, and discloses a preparation method of pure phase nanocrystalline yttrium doped bismuth ferrite. According tothe method, iron oxide, bismuth oxide and yttrium oxide are used as raw materials, the above raw materials are subjected to high-energy ball milling by a mechanical alloying method to obtain amorphousyttrium doped bismuth ferrite, and the amorphous yttrium doped bismuth ferrite is subjected to high-temperature heat treatment to produce the pure yttrium doped bismuth ferrite nanocrystalline structure. The yttrium doped bismuth ferrite prepared by the method has good multiferroic property and superior photo-catalytic effects.