The　effect　of　COD/N　ratios　and　pH　on　N2O　production　was　studied　in　sequencing　batch　reactors　(SBR).　The　initial　NO2--N　concentration　of　30　mg　N/L　was　obtained　by　adding　NaNO2,　the　COD/N　ratios　of　0,　1.5,　3　and　4.5　were　reached　by　adding　ethanol,　and　the　pH　were　controlled　at　6,　7,　and　8.　The　experimental　results　showed　that　low　COD/N　produced　more　N2O　with　the　maximum　N2O　generating　capacity　of　2.35　mg/L,　and　low　pH　conditions　increased　the　accumulation　of　N2O.　The　production　rate　of　N2O　reached　2.35×10-3mgN/(mgMLSS·L·h)　at　low　COD/N　and　low　pH.　There　were　two　reasons.　The　ability　of　N2O　reductase　for　competing　electrons　was　weak,　and　sufficient　electron　donors　were　favorable　for　the　reduction　of　N2O.　Low　pH　affected　microbial　metabolism,　and　the　N2O　reductase　were　inhibited　by　HNO2　(produced　by　H+　and　NO2-).　The　study　showed　that　sufficient　carbon　sources　and　alkaline　conditions　were　the　key　factors　to　reduce　the　accumulation　of　N2O　in　nitrite　denitrification　processes.