Abstract: The traditional sodium carbonate-zinc oxide semi fusion method causes little corrosion of the crucible and effects small interferences, but the analytical results of sulfur are unstable due to an incomplete break of the lattice structure of barite. The decomposition conditions during semi fusion that affect the conversion of sulfur are investigated and reported in this paper. Results show that the incomplete decomposition of barite was due to the grain sizes of samples and fluxes being too big. The modified method is that samples with sizes of less than 0.1 mm mixed with sodium carbonate-zinc oxide flux with sizes of less than 0.18 mm are decomposed at 790℃ for 60 min. Under these conditions, barite can be converted to BaCO₃ and isolated with SO₄^2- quantitatively through semi molten decomposition in a porcelain crucible, which avoids the use of the platinum crucible, damage of crucibles by strong oxidizing agents, and reduces the interference substances during melting. The RSD for this method was less than 1% and the recovery was more than 99.5%. This method can be applied to determine sulfur content of complex geological samples containing barite.