OBJECT IVE To prepar e and characterize danazol （DAZ）-sodium caseinate （SC）composite nanoparticles ，and to study the mechanism of preparing nanoparticles in “bottom-up”technology. METHODS SC was used as a stabilizer for regulating nanoparticles，so that DAZ-SC composite nanoparticles were prepared by anti-solvent precipitation method. The particle size ， Zeta-potential，micro-morphology，stability，encapsulation efficiency ，drug loading and in vitro dissolution rate were characterized. Fluorescence spectra ，IR spectra ，FBRM and other methods were used to analyze the interaction mechanism between DAZ and SC. RESULTS The particle size of DAZ-SC composite nanoparticles was （223.7±12.5）nm，and the polydispersity index was 0.274± 0.012. Zeta-potential was －（17.81±1.63）mV（n＝3）. The stability of nano-suspension was good ，the solid properties of DAZ were greatly improved ，and the dissolution rate was significantly increased. SC was statically quenched under the action of DAZ and the secondary structures of SC were changed. The crystallization process of DAZ was controlled under the action of SC ，and the interaction between DAZ and SC was mainly hydrogen bond and van der Waals force. CONCLUSIONS In this study ，DAZ-SC composite nanoparticles are successfully prepared. In the “bottom-up”technology，the interaction between SC and DAZ caused by hydrogen bond and van der Waals force inhibits the growth and agglomeration of drug crystals .