Abstract: The sulfur-containing gases evolution and the transformation of sulfur during pyrolysis of mild liquefaction solid product (MLS) were studied in a fixed-bed reactor. Meanwhile, the effects of mineral matters on the sulfur transformation were explored. The results show that most of the sulfur remains in the char under the experimental conditions in this work. Less than 10% of the sulfur migrates into tar and sulfur-containing gases. The sulfur-containing gases generated in pyrolysis is mainly H₂S. In addition, the generation rate of H₂S reaches the maximum when MLS is pyrolyzed at 400 ℃. The contents of various forms of sulfur in both MLS and its pyrolysis char were determined by the modified method. The analysis shows that the decomposition and conversion of sulfide sulfur and organic sulfur are the main reactions of sulfur-containing matters during the pyrolysis process of MLS. As the pyrolysis temperature increases, the organic sulfur in MLS gradually decomposes and is converted into sulfur-containing gases. When the temperature is lower than 600 ℃, the sulfide sulfur in MLS is gradually transformed into sulfur-containing gases, organic sulfur and a small amount of pyrite sulfur. When the pyrolysis temperature is higher than 600 ℃, the alkaline minerals in MLS would absorb H₂S and convert into sulfide sulfur. Consequently, the content of sulfide sulfur slowly increases. Acetic acid pickling treatment can retain most of the sulfide sulfur in MLS. After pickling, the generation rate of H₂S during MLS pyrolysis increases, and the peak temperature shifts to the lower temperature. When the pyrolysis temperature is over 600 ℃ the desulfurization reaction rate of the organic sulfur and sulfide sulfur decreases. Meanwhile, the alkaline minerals in MLS can react with H₂S to form metal sulfides, leading to a significant decrease in H₂S generation rate.