A simultaneous nitrification and denitrification (SND) process in sequencing batch biofilm reactor (SBBR) was established to treat high carbon and nitrogen saline wastewater in this study. Acetate, glucose and an organic mixture were applied as organic sources in three SBBRs, achieving average total nitrogen removal efficiency of 97.15%, 63.94% and 94.99% during 120 days' operation, respectively. The underlying nitrogen removal mechanisms were investigated by 16S rRNA sequencing and batch tests. Results showed different carbon sources had great impact on microbial communities, and led to different nitrogen removal mechanism. Autotrophic and heterotrophic nitrification together contributed to the well performance of nitrification process. And denitrification was carried out by a combined anoxic and aerobic denitrificans. Furthermore, the SND process was mainly via nitrite not nitrate. Compared with ammoniaoxidizing bacteria, ammonia-oxidizing archaea with a much higher abundance contributed more to autotrophic nitrification. Pseudomonas_stutzeri and Bacillus_cereus were the predominant detected heterotrophic nitrification-aerobic denitrification bacterium. (C) 2017 Elsevier Ltd. All rights reserved.