The gel formation and transformation to crystal of moxidectin (Form I) during the anti-solvent crystallization was studied in this paper. Several characterization techniques were used to monitor and identify this process, including online and offline instruments, such as X-ray powder diffraction (PXRD), polarizing microscope(POM), differential scanning calorimetry (DSO), focused beam reflectance measurement (FBRM) and On-line Forier transform infrared (FTIR). The results demonstrated that tiny crystal nucleus could be found after certain amount water was added into the solution, and then the tiny crystal nucleuses served as gelators and induced the system to form gel through the hydrogen bonds between moxidectin and water molecules. Finally, the gel would transform to form I due to the instability of gel.