The　feasibility　of　stable　nitritation　of　sludge　alternately　treated　by　free　nitrous　acid　(FNA)　and　heat　shock　in　a　sequencing　batch　reactor　(SBR)　was　investigated　in　this　study.　The　linear　regression　method　was　used　to　determine　the　optimal　treatment　conditions.　Results　revealed　that　an　FNA　concentration　of　2.20　mg　HNO2-N/L,　exposure　time　of　24　h,　and　treatment　ratio　of　20%　could　inhibit　nitrite　oxidizing　bacteria　(NOB)　activity　to　the　greatest　extent　while　maintaining　the　maximum　ammonium　oxidizing　bacteria　(AOB)　activity;　after　heat　shock　at　60　?&　nbsp;for　20　min,　NOB　were　inhibited　while　AOB　still　had　certain　activity.　In　the　long-term　continuous-flow　experiment,　the　single　FNA　or　heat　shock　treatments　easily　allowed　adapt　NOB　to　affect　the　stability　of　nitritation.　The　alternating　FNA/heat　shock　treatment　can　achieve　long-term　stability　of　nitritation.　Microbial　community　analysis　revealed　that　the　alternating　FNA/heat　shock　treatment　could　inhibit　NOB　while　maintaining　high　AOB　abundance.