Microbial activities of the sludge taken from the sequencing batch reactor (SBR) were measured by batch experiments under salinity stress, and the impacts of salinity on nitrifying bacteria and denitrifying bacteria were analysed. The special ammonia oxidation rate (SAOR:0~30gNaCl/L), special nitrite oxidation rate (SNIOR:0~15gNaCl/L), special nitrite denitrication rate (SNIDR:0~35gNaCl/L), special nitrate denitrification rate (SNADR:0~40gNaCl/L), endogenesis special nitrite denitrification rate (E-SNIDR:0~25gNaCl/L), endogenesis special nitrate denitrification rate (E-SNADR:0~5gNaCl/L), special oxygen uptake rate (SOUR:0~30gNaCl/L) and dehydrogenase activity (DHA:0~80gNaCl/L) were conducted for activities analysis in this study. The results indicated that when salinity was 5gNaCl/L, salt stress could enhance SNIDR and SOUR. The activity cofficients (AC) of SAOR and SNIOR were decreased to 46% and 1% under salinity of 15gNaCl/L, respectively, so compared with ammonia oxidizing bacteria, nitrite oxidizing bacteria was more easily inhibited by salt stress. The ACs of SNIDR and SNADR were higher than SAOR and SNIOR when salinity higher than 10gNaCl/L (included 10gNaCl/L) showed that denitrifying bacteria had more salt tolerance than nitrifying bacteria. DHA could still be detected at NaCl concentration as high as 80g/L, indicating that microorganisms still had the ability of catalyze redox reaction.