Abstract: Sulfate-reducing anaerobic ammonia oxidation (SRAO) reaction can be used to remove nitrogen and sulfate simultaneously in the same process, providing a new approach for complex industrial wastewater treatment in wastewater with a high concentration of ammonia and sulfate. However, the unstable SRAO depends easily on various external environmental factors. Particularly the main substrate, the concentrations and their ratios of ammonia nitrogen and sulfate are of great importance to the SARO process. In this study, a systematic investigation was made on the effect of the S/N (NH4+-N/SO42-) ratio at different concentrations on the removal of nitrogen and sulfur in an anaerobic sequencing batch reactor (ASBR). Two ASBRs with a volume of 2.0 L were also used in parallel in the experiment, where the hydraulic retention time was 48 h. The ASBRs were wrapped up with the cotton layer and black paper to isolate from the environment, while remaining at 30 ℃ with the hot water from the water-bath with the casing layer of reactors. The four stages of reactors included loading, mixing, precipitation and drainage, which were controlled by automatic program controllers. The ratio of nitrogen to sulfur was adjusted by adding different concentrations of ammonium chloride and sulfate. The results showed that when N/S increased from 1.0 to 3.0 under the condition of sulfate concentration of about 100 mg/L, the NH4+ removal increased from 84.4% to 94.4%. The high-throughput sequence analysis showed that relative abundance of Candidatus Kuenenia in the anaerobic reactor increased from 8.6% to 10.6%, and the relative abundance of Candidatus Brocadia increased from 15% to 17.4%. At this time, the relative abundance and activity of Sulfur Autotrophic Denitrification (SAD) bacteria in ASBR did not change significantly, but the relative abundance of sulfate-reducing ammonia oxidation functional bacteria Candidatus Kuenenia and Candidatus Brocadia increased significantly to 13.9% and 20.7%, respectively. The sulfate reduction ammonia oxidation (SRAO) and ANAMMOX were considered to be the main ways of nitrogen removal. An obvious decrease of NH4+ removal from 94.4% to 69.2% when the S/N increased from 3.0 to 4.0, which showed inhibition of SRAO and ANAMMOX bacteria in higher S/Ns over 3.0. At the same time, the content of thioplaca (Sulfur autotrophic denitrifying bacteria) and nitrifying bacteria remained unchanged. The relative abundance of Candidatus Kuenenia and Candidatus Brocadia decreased to 8.5% and 16.5%, respectively. Combined with the experimental results, the removal of sulfate according different S/Ns showed a similar trend as TN removal. The TN and sulfur removal rate could reach 94.4% and 74.2% when the N/S value was 3.0. High-throughput sequencing analysis showed the different dominant species of bacteria at the S/Ns, indicating the variation of TN and sulfur removal rate in the ASBR. The sulfate removal at different N/S ratios demonstrated combined denitrification in the system, including SRAO, SAD, and traditional ANAMMOX. The SARO was the dominant process of denitrification, when N/S<1.5, whereas, the effect of ANAMMOX was the main denitrificaiton, when N/S>1.5. An optimum N/S of 3.0 can be expected to effectively couple the SARO and ANAMMOX for the greatest nitrogen removal.