Aiming at the influence to municipal sewage treatment in the utilization of seawater, effects of salinity on nitrogen removal were studied using A/O process. The experiment indicated that the salinity of 20g/L was the tolerance limit for the nitrogen removal of the system. The salinity within 10g/L had little effect on the removal efficiency of ammonia nitrogen. At 20g/L salinity, the removal rate still kept high value and above 80% after the sludge being domesticated. The treatment efficiency plunged and the ammonia nitrogen removal rate dropped to 50% under 25g/L salinity condition. The nitrification rate decreased with the increase of salinity and changed little at the salinity from 5 g/L to 15 g/L. Under the salinity above 20 g/L conditions, it decreased evidently. Otherwise, the proper salinity was beneficial to shortcut nitrification-denitrification. The shortcut nitrification-denitrification was realized at the salinity above 15 g/L. The nitrite-nitrification rate was 97% at 20 g/L salinity.