Abstract: To resolve the yellow phase problem, it is necessary to understand sulphur behaviors in the borosilicate waste glasses, especially the sulfate phase segregation behavior and its main decomposition temperature. The sulfate phase segregation and decomposition in simulated high level liquid waste (HLLW) vitrification process were studied in this paper. The results indicate that molten sulfate segregation happens at early stage (about 900 ℃) of the batch melting. Once sulfate is segregated from the melt, it is difficult to be redissolved. The sulfate in yellow phase begins to decompose at 800 ℃, and decomposition is speeded up with the increase of temperature. After 2 h at 1 150 ℃, no yellow phase is found at the melt surface, and sulfur content retained in the melt doesn’t change any more. Based on the research result, the methods for eliminating yellow phase were proposed and verified.