Coal supercritical water gasification (SCWG) is famous for generating clean gas without SOx pollutant. Study of sulfur transformation characteristics can provide the basis of sulfur removal during hydrogen production by coal gasification in supercritical water (SCW) at the source. In this work, two coals produced from Linfen and Zhangjiamao in China (hereinafter to be referred as L-coal and Z-coal), were chosen as experimental feedstocks to investigate sulfur transformation characteristics during hydrogen production by coal gasification in SCW (550-750 degrees C, 20 min, 25 MPa). Sulfur transformation pathway and sulfur forms in the products were complex but detected comprehensively. H2S was the only gaseous product instead of SOx, whereas SO42- was the main liquid sulfur product. Inorganic and organic sulfur compounds were used to investigate sulfur transformation mechanisms. H2S had three sources as follows. First, among inorganic sulfur of raw coal, FeS2 (Pyrite) was chemically stable in SCW lacking of hydrogen. When FeS2 was in hydrogen atmosphere, H2S was generated and FeS2 was converted to Fe1-xS and Fe3O4 under SCW. Second, H2S came from unstable sulfate minerals such as FeSO4 which may decompose and be converted to Fe3O4. Third, organic sulfur, especially thiophene sulfur transformed to H2S. The two sulfur products H2S and SO42- depend on H or OH free radical in SCW. More H free radical provided a reducing environment of SCW to generate H2S at higher temperatures, whereas more OH radical provided an oxidizing environment of SCW to generate SO42- at lower temperatures, but the final trend was generating H2S when coal gasified completely at a higher temperature. The results of this study may provide an experimental basis of solving the SOx emission from coal at the source and demonstrate a promising clean utilization way of coal.