Reactive force field molecular dynamics (ReaxFF-MD) (reactive force field molecular dynamics) is a promising method for exploring complex chemical reactions, allowing a better understanding of sulfur transformation during coal pyrolysis. In this work, we built three pyrolysis systems with different H2O contents to explore the effect of H2O on the transformation of sulfur during demineralized pyrolysis by ReaxFF-MD. Product distributions, sulfur-containing bonds, and the path of organic sulfur in different systems were analyzed. We found that H2O could accelerate the pyrolysis process, reduce the semicoke yield, and increase the production of gas components significantly, including H2S gas. By adding H2O during pyrolysis, more sulfur in semicoke and heavy tar converted to gas and light tar, respectively. H2O not only promoted the cleavage of C-S bonds but also generated more H free radicals to form SH free radicals, thereby promoting the removal of organic sulfur.