Taking dimethyl sulfoxide as solvent， soy isoflavone powder was prepared by acetone as the anti-solvent. Orthogonal experimental design method was used to in spect the major factors affecting the size and morphology of soy isoflavone powder. Results show that the particle size has a negative relationship with volume ratio， rotor speed and concentration， and has a positive relationship with reaction time. Experiment indicates that the measured powder particle size reaches 98 nm when the concentration is 300 mg/mL； rotor speed is 1 200 r/min； volume ratio is 1 ∶ 6 and reaction time is 3 min. Isoflavone and nano-isoflavone power were evaluated by using scanning electron microscope （SEM）， infrared spectroscopy （FT-IR）， differential scanning calorimetry （DSC） and statistics of powder heap density. Results showed that the chemical properties of isoflavones did not change after preparation. Nano-isoflavone particles were distributed evenly and the particle size decreased significantly.