Accumulation of nitrites is frequently observed in saline wastewater nitrification processes, which often results in the release of the strong greenhouse gas, nitrous oxide (N2O). This study investigated the effect of salinity on microbial activity and N2O release characteristics during the simultaneous nitrification and denitrification processes in a sequencing batch biofilm reactor (SBBR). The result showed that salinity inhibited the microbial activities of each bacterial group in increasing, sequential order as follows: nitrite oxidizing bacteria (NOB)>ammonia oxidizing bacteria (AOB)>carbon oxidizing bacteria. The effluent COD was stable at about 50.0 mg/L in the range of salinity from 0 to 20 g NaCl/L. The average NH4+ removal efficiency reduced from more than 98% to 70.5%, and TN removal efficiency reduced from 42.4% to 16.9% respectively, while the N2O yield increased from 3.9% to 13.3%. Similar to SND efficiency, the internal concentration of carbon sources (PHA and Gly) first increased and then decreased with increase in salinity. The N2O release was mainly produced in AOB aerobic and endogenous denitrification processes. The low N2O release could be ascribed to the anoxic zone deep in the SBBR at low salinity (≤10 g NaCl/L). As the salinity was increased to more than 10 g NaCl/L, there was a decrease in internal carbon source synthesis, which aggravated the electron competition between each bacterial reductases in the denitrification process. Further increase in salinity led to an increase in extracellular polymer substances (EPS) synthesis and the proportion of polysaccharides (PS) in the EPS. A decrease in the anoxic area deep in the membrane led to the inhibition of the N2O reduction.