A　lab-scale　Cyclic　Activated　Sludge　Technology　(CAST)　system　was　operated　more　than　5　months　to　evaluate　the　effects　of　the　operation　mode　on　nitrogen　removal　performance　and　investigate　a　feasible　method　for　achieving　short-cut　nitrification　in　the　system.　Results　showed　that　nitrogen　was　removed　by　conventional　biological　nitrification　and　denitrification　in　traditional　operation　mode　(fill/aeration　2　h,　settle　1　h,　decant　1　h),　whereas　short-cut　nitrification　and　denitrification　was　the　main　nitrogen　removal　pathway　in　modified　operation　mode　and　the　nitrogen　removal　performance　was　enhanced.　Short-cut　nitrification　was　successfully　achieved　in　CAST　system　at　17　+/-　1　degrees　C　by　adjusting　operation　conditions　and　the　average　total　nitrogen　removal　efficiency　increased　by　11.4%　compared　to　traditional　mode.　It　was　assumed　that　low　dissolved　oxygen　(＜　1.0　mg/L)　limitation　combined　with　free　ammonia　(0.28-0.34　mg/L)　inhibition　on　nitrite-oxidizing　bacteria　caused　nitrite　accumulation　in　modified　mode.　During　maintaining　period　of　short-cut　nitrification,　preset　aeration　time　enhanced　ammonium-oxidizing　bacteria　dominance.　It　was　also　found　that　low　DO　could　result　in　overgrowth　of　filamentous　microorganisms　and　poor　sludge　settleability.　The　pH　variation　could　provide　effective　information　for　controlling　aeration　duration　in　modified　mode.　However,　no　evident　breakpoint　appeared　on　pH　and　DO　profiles　in　traditional　mode.