Abstract: There is a coupling of sulfur cycle with denitrification (DN) and dissimilatory nitrate reduction to ammonium (DNRA) in the sediment-water system. The effects of reduced sulfur on DN and DNRA were revealed by sequentially adding 0 (Group N), 50 (Group L) and 75 mg·L^-1 (Group H) reduced sulfur to the constructed sediment-water microcosm systems, and then the concentration changes of various forms of nitrogen in overlying water, pore water and sediment as well as microbial community structure were analyzed. Results show that 38.02% and 33.27% of reduced sulfur were converted into SO₄^2-, respectively, when the dosage of reduced sulfur was 50 and 75 mg·L^-1. Sulfur oxidation promoted the occurrence of DN, and caused the total nitrogen of Group L and Group H decrease by 11% and 8%, respectively, compared with Group N, but had no significant effect on the process of DNRA. In addition, the sum of relative abundances of genera Thiobacillus and Dechloromonas functioned with sulfur autotrophic denitrification increased with concentration of reduced sulfur, and occupied 5.69%, 6.52%, 8.70% in Group N, L and H, respectively. In contrast, the relative abundance of heterotrophic nitrate reducing bacteria (HNRB) was only 0.98%, 0.91%, and 1.05% in Group N, L and H, respectively, and the DNRA bacteria (with the relative abundance no more than 1%) were unaffected by reduced sulfur. Such results further confirmed that the nitrate reduction process in the system was dominated by sulfur autotrophic denitrification.