The　control　strategy　and　simulation　of　external　carbon　addition　were　specially　studied　in　an　anoxic-oxic　(A/O)　process　with　low　carbon:　nitrogen　(C/N)　domestic　wastewater.　The　control　strategy　aimed　to　adjust　the　flow　rate　of　external　carbon　dosage　to　the　anoxic　zone,　thus　the　concentration　of　nitrate　plus　nitrite　(NOx(-)　-N)　in　the　anoxic　zone　was　kept　closed　to　the　set　point.　The　relationship　was　studied　between　the　NOx(-)　-N　concentration　in　the　anoxic　zone　(S-NO)　and　the　dosage　of　external　carbon,　and　the　results　showed　that　the　removal　efficiency　of　the　total　nitrogen　(TN)　could　not　be　largely　improved　by　double　dosage　of　carbon　source　when　S-NO　reached　about　2　mg/L.　Through　keeping　S-NO　at　the　level　of　about　2　mg/L,　the　demand　of　effluent　quality　could　be　met　and　the　carbon　dosage　could　be　optimized.　Based　on　the　Activated　Sludge　Model　No.　1　(ASM　No.　1),　a　simplified　mathematical　model　of　external　carbon　dosage　was　developed.　Simulation　results　showed　that　PI　controller　and　feed-forward　PI　controller　both　had　good　dynamic　response　and　steady　precision.　And　feed-forward　PI　controller　had　better　control　effects　due　to　its　consideration　of　influent　disturbances.