In　step-feed　A/O　biological　nitrogen　removal　system,　the　optimal　influent　flow　rate　distribution　can　make　full　use　of　the　carbon　source　in　influent,　and　the　appropriate　anoxic/oxic　volume　ratio　can　save　the　operation　cost　and　resist　the　influent　disturbance　effectively.　Based　on　the　characteristics　of　configuration　and　operation　of　step-feed　process,　this　paper　analysed　the　influent　flow　rate　and　volume　distribution　theoretically　and　discussed　the　optimization　of　operation.　Regarding　to　the　influent　flow　rate　distribution,　the　influent　C/N　ratio　was　employed　as　a　control　parameter,　and　the　distribution　method　under　three　different　conditions　when　the　amount　of　carbon　source　was　sufficient,　exactly　appropriate　and　insufficient　respectively,　was　discussed.　Then　the　methods　were　given　to　optimize　the　operation.　In　addition,　the　effect　of　volume　distribution　on　the　capacity　of　nitrification　and　the　utilization　of　COD　in　influent　was　also　analyzed　and　the　optimization　of　system　under　the　different　loads　was　discussed.