During vitrification of high-level liquid waste with high content of sulfur and sodium, Na2SO4 is very easy to decompose and separate from the glass melts. In this paper, it was proposed that Na2SO4 in the high-level liquid waste was converted into PbSO4 by addition a proper amount of Pb(NO3)2 solution, and then the borosilicate glass solidified body was prepared by melting method. The dissolution characteristics of PbSO4 in glass melt was observed in situ for the first time by high temperature confocal scanning laser microscope, which has been widely used in steel materials. The thermal stability of PbSO4 mixed with borosilicate glass at different temperatures (800 ℃ to 1 150 ℃) and the sulfur content in glass were investigated. The results show that borosilicate glass samples with 6% (mass fraction) SO3 introduced in the form of PbSO4 are homogeneous glass-ceramics at both 800 ℃ and 900 ℃. The glass-ceramics mainly possess SiO2 phase along with a small amount of BaSO4 and PbSO4 phases at 800 ℃. The content of SiO2 crystal decreases while the BaSO4 crystal content increases, and PbSO4 phase disappears and CaMgSi2O6 phase appears at 900 ℃. A white separated phase begins to appear on the glass surface composed of PbO, BaSO4 and LiNaSO4 crystals at 1 000 ℃. In addition, the round shape PbO crystals grow gradually and BaSO4 crystals change from block to strip shape with the temperature increasing from 1 000 ℃ to 1 100 ℃. The sulfur content in the glass remains unchanged at 800 ℃ to 1 000 ℃, and decreases obviously with further increasing the temperature.