The circulating-fluidized-bed bioreactor (CFBBR) is a promising technique for biological-nutrient removal (BNR) from wastewater. A simulation-based method for determining the hydraulic-retention time (HRT) and operating conditions of a CFBBR is proposed. When a CFBBR operates under the constant HRT and the constant operating conditions obtained by the proposed method, the effluent can meet the water-quality criteria in cases with composition disturbances and implementation errors. To reduce the HRT, two ways of adding a carbon source are investigated: (i) increasing the minimum carbon-nitrogen ratio of the fluctuating influent and (ii) adding a constant amount of a carbon source. The relationships between the minimum carbon-nitrogen ratio (TCOD increment) and the minimum HRT are obtained. These relationships are helpful for guiding the addition of a carbon source. A case study of a pilot-scale CFBBR for BNR is used to illustrate this method.