In　order　to　explore　the　best　combination　of　process　parameters　related　to　the　strategy　of　inhibiting　nitrite　oxidizing　bacteria　(NOB)　by　side　flow　free　nitrous　acid　(FNA)　treatment,　in　the　completely　autotrophic　nitrogen　removal　over　nitrite　(CANON)　process　of　granular-flocculate　composite　system,　batch　test　was　used　to　explore　the　effects　of　sludge　sedimentation　time,　FNA　treatment　concentration　and　FNA　processing　and　processing　time　to　the　related　functional　bacteria　activity　influence.　The　NOB　activity　was　targeted　inhibition　and　treatment　time　on　the　activity　of　relevant　functional　bacteria,　so　as　to　inhibit　the　activity　of　NOB　and　reduce　the　effect　of　FNA　on　the　activity　of　functional　bacteria.　The　result　showed　that　the　settling　time　affected　the　activity　of　functional　bacteria　in　the　treated　sludge.　With　the　prolongation　of　the　settling　time,　the　activity　of　anaerobic　ammonium　oxidizing　bacteria　(AnAOB)　in　the　discharged　sludge　gradually　decreased.　After　settling　for　1min,　most　of　the　discharged　sludge　was　NOB,　and　retained　as　much　AnAOB　as　possible　in　the　reactor.　At　this　time,　the　relative　activities　of　AnAOB,　ammonia　oxidizing　bacteria　(AOB)　and　NOB　were　15.79%,　54.55%　and　68.63%,　respectively.　Considering　the　effect　of　FNA　on　the　activity　of　NOB　and　AOB,　the　activity　of　AOB　was　38.71%,　while　the　NOB　activity　was　only　12.5%　after　inhibition　with　0.6mg/L　FNA　for　12h.　The　results　of　response　surface　analysis　showed　that　the　treatment　time　and　treatment　concentration　of　FNA　were　the　key　factors　affecting　the　activity　of　NOB　and　AOB.　©　2022　Chinese　Society　for　Environmental　Sciences.　All　rights　reserved.