OBJECTIVE： To prepare the betulinic acid nanoparticles， and characterize them. METHODS： Using ethanol as solvent and water as anti-solvent， anti-solvent recrystallization method was used to prepare betulinic acid nanoparticles. Using particle size as indicator， single factor test and orthogonal test were adopted to optimize the mass concentration of betulinic acid solution， anti-solvent-solvent volume ratio， anti-solvent drip rate， reaction temperature and stirring speed in formulation technology of betulinic acid nanoparticles， and verification test was conducted. The betulinic acid nanoparticles were characterized by scanning electron microscopy， laser particle size analyzer， Fourier infrared spectrometer and mass spectrum analyzer. RESULTS： The optimal technology was as follow as betulinic acid solution mass concentration of 3 mg/mL， anti-solvent-solvent volume ratio of 1 ∶ 1， anti-solvent drip rate of 8 mL/min， reaction temperature of 20 ℃ and stirring speed of 900 r/min. The average size of prepared betulinic acid nanosuspension was （156.0±8.6） nm （n＝3） and the particle size was （235.0±12.2） nm （n＝3） after freeze-drying， with nearly spherical appearance， uniform size and regular form. Compared with raw material of betulinic acid， the chemical structure of prepared betulinic acid nanoparticles did not change， and there were no significant changes in molecular weight and mass ratio. CONCLUSIONS： Betulinic acid nanoparticles are successfully prepared.