Carbon-to-nitrogen ratio （C/N） is one of the most important factors affecting aerobic denitrification effect. Based on the cyclic activated sludge system （CASS） process with low-carbon （C/N=1.8） influent， the effects of drainage ratio， aeration time， and water intake mode on denitrification effect are investigated separately. The results show that limited by the influent carbon source concentration， the drainage ratio is the main factor affecting the nitrogen removal efficiency. The average total nitrogen （TN）concentration of CASS effluent was 17.8 mg·L^-1， 9.5 mg·L^-1 and 6.6 mg·L^-1 when the drainage ratio was 30%， 40% and 50%， respectively； Therefore， for CASS process， the drainage ratio should be adjusted appropriately according to the carbon nitrogen ratio instead of being set to 30%.