The vitrification of highlevel liquid waste (HLLW) is one of the challenges in the reprocessing of spent nuclear fuel in China. The feedtoglass conversion comprising quite complex chemical reactions and structure transitions has been of great concern. In this paper, the phase and structure transitions of the heattreated feed (mixture of simulated HLLW calcine and glass frit) during vitrification were analyzed with DSC, insitu XRD, NMR and Raman. The results show that the simulated HLLW calcine begins to dissolve in the glass frit at ~700 ℃, and the intermediate phases of aluminosilicates appeare at 800~900 ℃. During the reactions of HLLW calcine and glass frit at elevated temperatures, most of network modifiers from HLLW enter into the glass network of glass frit, resulting in a decrease of the network polymerization.