The invention discloses a preparation method for rare-earth-doped borate nanosheet or nanoflower fluorescent powder. The preparation method comprises the steps as follows: (1) dissolving nitrate in a mixed solution of de-ionized water and ethanol; (2) adding tributyl borate, wherein the addition amount of tributyl borate in the solution is 1-4 times that of nitrate; (3) adding ammonia water and adjusting the pH value of the solution to be 7.5-8.5, wherein the ratio of the total volume of water and the ammonia water to the volume of ethanol in the solution is 1 to 2 and the ratio of the addition amount of nitrate to the total volume of the water and ethanol is 0.025-0.1 mol/L; (4) stirring the solution for 1-2 hours and insulating for 10-16 hours at the temperature of 160-200 DEG C to obtain white deposits; and (5) centrifuging, washing and drying the white deposits to obtain sheet-shaped or flower-shaped rare-earth-doped yttrium borate fluorescent powder. According to the preparation method, the operation is easy, the process equipment is simple, a toxic solvent is not used, and the prepared nanosheet or nanoflower is uniform in thickness, good in dispersity and controllable in structure.