Hollow　anaerobic　ammonium　oxidation　(Anammox)　granular　sludge　formed　and　became　the　main　part　in　the　treatment　of　low-strength　ammonia　wastewater.　The　characteristics　and　formation　mechanism　of　hollow　Anammox　granular　sludge　were　investigated　in　a　continuous　stirred　tank　reactor　(CSTR)　at　room　temperature.　Granular　sludge　was　found　to　be　cavitated,　and　Candidatus　Kuenenia　was　the　dominant　bacteria　that　lived　and　coexisted　with　other　bacteria　in　walls　under　an　extracellular　polymeric　substance　(EPS)　shield.　The　EEMPARAFAC　analysis　was　applied　to　characterize　the　EPS,　while　EPS　secretion　was　the　expression　of　specific　Anammox　activity.　Tryptophan　or　protein-like　was　the　active　component　of　proteins　(PN)　as　the　main　part　of　EPS　in　granules.　These　results　confirm　that　hollow　Anammox　granular　sludge　is　suitable　for　low-strength　ammonia　sewage.　Finally,　a　conceptual　model　of　hollow　Anammox　granules　growth　and　breakage　was　developed,　and　cavitation　was　a　key　step　of　regranulation.