This　study　investigates　the　effects　of　pH　on　the　stability　and　activity　of　ANAMMOX　granules　with　different　biomass　densities　from　the　perspective　of　N-acyl-homoserine　lactone　(AHL)-based　quorum　sensing　(QS).　Results　clearly　showed　that　the　effects　of　pH　on　the　release　of　AHLs　in　ANAMMOX　granules　and　the　properties　of　the　granules　are　closely　related　to　biomass　density　under　neutral　and　weakly　acidic　conditions　(pH　6.0-7.0).　Under　these　conditions,　pH　had　greater　adverse　effects　on　the　granules　with　low　biomass　density　(L-granules)　than　the　granules　with　high　biomass　density　(H-granules).　When　pH　decreased　from　7.0　to　6.0,　the　H-granules　were　able　to　release　abundant　AHLs　(the　release　amounts　of　C6-HSL　and　C8-HSL　reached　up　to　1482.4　and　1194.7　ng　L-1,　respectively,　at　pH　6.0),　which　further　promoted　extracellular　polymeric　substance　(EPS)　secretion,　serving　to　maintain　the　stability　and　activity　of　the　granules　under　neutral　and　weakly　acidic　conditions.　But,　L-granules　could　not　release　enough　AHLs,　leading　to　the　low　EPS　content　and　deterioration　in　the　stability　and　activity　of　the　granules　under　these　conditions.　However,　under　neutral　and　weakly　acidic　pH　conditions,　the　generation　mechanism　of　AHLs　in　L-granules　could　be　activated　by　short-term　(10　d)　exogenous　addition　of　C6-HSL　and　C8-HSL,　causing　excessive　endogenous　release　of　C6-HSL　and　C8-HSL,　which　was　conducive　to　improving　the　stability　and　activity　of　L-granules　under　adverse　weakly　acidic　pH　conditions.　Under　peracid　(pH　5.5)　and　alkaline　(pH　7.5-9.0)　conditions,　the　effects　of　pH　on　AHL　release　and　granule　characteristics　were　not　related　to　biomass　density.　At　pH　5.5,　the　QS　system　of　the　granules　was　destroyed,　and　the　release　amounts　of　AHLs　in　H-granules　and　L-granules　were　very　low,　leading　to　poor　stability　and　low　activity　of　the　granules.　Under　alkaline　conditions　(pH　7.5-9.0),　the　release　amounts　of　AHLs　in　H-granules　and　L-granules　were　significantly　reduced　with　the　increase　in　pH,　and　their　stability　and　activity　also　decreased　significantly,　which　was　related　to　the　degradation　of　AHLs　under　alkaline　conditions.