Abstract: To investigate the vital role of complex microbial communities in activated sludge about the pollutant removal in wastewater treatment plants (WWTPs), metagenomic sequencing on Illumina HiSeq 4000 platform was applied to characterize microbial community, functional profiles and metabolic pathway within activated sludge from a full-scale municipal WWTP. The microbial community was conducted taxonomic and function annotations based on KEEG and COG database. Taxonomic analysis show that the dominant bacterial phyla are Proteobacteria, Bacteroidetes, Nitrospirae, Actinobacteria Chloroflexi and Firmicutes(the abundance are 53.6%, 25.3%, 5.86%, 2.43%, 1.71%, and 1.46%, respectively, of the overall population reads). The key organisms involve Nitrosomonas, Nitrospira and Thauera (approximately 5.82%, 2.26% and 4.30%, respectively), which are the typical ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB) bacteria and denitrifying bacteria. These functional microorganisms carry out biological nitrogen removal (BNR). Various key enzymes involved in the global nitrogen cycle are annotated in the activated sludge. The abundance of the functional genes in the BNR is quantified (amo:1 966 hits, hao:1 000 hits, narG:8 204 hits, napA:1 828 hits, nirk:1 854 hits, norB:2 538 hits, and nosZ:5 158 hits).The function genic abundance of nitrite oxidordeuctase is 8 248 hits. This study provides a comprehensive insight into the community structure and diversity of the BNR system, and will provide foundation for optimal operation of nutrient removal systems.