Controlling　of　solids　retention　time　(SRT)　in　the　activated　sludge　process　is　critical　for　ensuring　effective　wastewater　treatment.　Because　of　the　disadvantages　of　conventional　control　methods,　the　hydraulic　method　of　controlling　SRT　was　improved　in　this　study　and　applied　into　the　step-feed　biological　nitrogen　removal　process.　After　the　application　of　the　hydraulic　control　method,　conventional　SRT　changed　from　17.67　to　19.08　days　and　hydraulic　SRT　was　constant　at　a　desired　20　days.　Hydraulic　control　of　SRT　simplified　process　control,　and　produced　good　effluent　quality　where　the　total　nitrogen　removal　efficiency　with　four　stages　ascended　from　72.6　to　85%.　Variations　of　hydraulic　SRT　under　different　influent　wastewater　strength　and　flow　rates　were　investigated,　and　the　results　showed　that　a　relative　constant　SRT　value　could　be　maintained　easily.　The　feasibilities　of　applying　the　hydraulic　method　of　controlling　SRT　in　step-feed　biological　nitrogen　removal　process　with　different　operational　modes,　including　different　stage　numbers　processes,　different　influent　flow　rate　distributions,　and　volumes　in　each　stage　and　different　volume　ratios　of　the　anoxic　zone　to　the　aerobic　in　each　stage,　were　verified　and　the　results　were　satisfactory.　Under　constant　SRT　the　total　nitrogen　removal　efficiency　of　the　step-feeding　process　with　three,　four,　and　five　stages　was　80,　85,　and　95%,　respectively.