Anaerobic ammonia oxidation (Anammox) faces challenges in high salinity environments due to inhibited microbial activity, whereas upflow anaerobic sludge bed (UASB) reactors mitigate this limitation by maintaining higher biomass concentration, which enhances microbial retention under osmotic stress. To explore how the Anammox-USAB system responds to the high salinity (NaCl) environment, a UASB reactor seeded with heterotrophic nitrification sludge. The salinity was gradually increased from 0 to 40 g NaCl/L. The results show that, when salinity increased from 0 to 15 g NaCl/L, the conversion rate of ammonia nitrogen (NH4+-N) and total nitrogen (TN) decreased by about 25% and 22 %, respectively. At the same time, the ammonium removal load of unit sludge gradually stabilized at about 3.65 mg NH4+-N/g VSS over 10 g NaCl /L. When the salinity gradient increased to 30 g NaCl/L, microorganisms preferentially increased their polysaccharide (PS) content from 5.50 to 8.26 mg/g VSS to resist the high osmotic pressure environment. Notably, extracellular protein increased significantly, from 5.78 to 29.01 mg/g VSS to stabilize the cell structure and maintain metabolic activities at 40 g NaCl/L. With the increase of salinity, some salt-intolerant bacteria were inhibited or killed, resulting in a continuous decline in abundance while the abundance of salt-tolerant bacteria increased. The abundance of dominant species Candidatus Kuenenia and Halomonas increased from 5.97% and 0.53% to 12.29% and 12.17%, respectively. It could be seen that the Anammox-USAB system used the structural adjustment of the microbial itself and community as an adaptive strategy in response to the changes of the high-salinity environment.