The feed-to-glass conversion within the cold cap and the mathematical modeling of the process are of great concern for the accurate simulation of the melter for vitrification of high-level liquid waste (HLLW). To figure out the feed-to-glass conversion during the HLLW vitrification, the phase transitions and structural features within the cold cap were investigated. The results show that the cold cap consists of the upper reaction layer (at 100-700 ℃) and the lower foam layer (at 700-1 000 ℃). The dehydration and nitrate decomposition occur in the reaction layer, while the continuous melt as well as the formation and dissolution of intermediate phases appear in the foam layer. The reaction layer can be simplified into a one-dimensional mass transfer model based on the mass conservation equation and experimental results. Subsequently, the kinetic equation of feed-to-glass conversion is proposed to quantitatively describe the three reaction rates of dehydration (at 100-400 ℃), and melting (at 200-300 ℃) and decomposition (at 400-700 ℃) of nitrates as functions of temperature in the reaction layer, providing some crucial parameters for melter simulation.