The volume Bragg grating was prepared by using the "five-step" thermal fixing process, crystallization mechanism of the photo-thermo-rraphictive glass is described, and the ion migration process and NaF crystallization process are intuitively detected by Raman spectroscopy and scanning electron microscopy. The absorption band at 350~600 nm is found by transmission spectrum, and the colloid formed in the internal nucleation process is determined to be a silver bromide and silver nanocomposite.The key factors influencing the crystallized particles in photo-thermal-refractive glass were investigated from three aspects:nucleation process, NaF crystallization process and UV exposure dosage. It is found that the size of[Ag_n⁰·(AgBr)_m] micelles is the key to determine the crystal size of NaF during nucleation. The crystallization process mainly affects the number of crystal particles and has little effect on changing the crystal size.As the exposure dosage increases, the crystallization size of NaF decreases first and then increases, the inflection point is around 1.8 J/cm², and the crystalline particles are the smallest.