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Selective catalytic reduction (SCR) is a promising technology for diesel aftertreatment used to reduce NOX emission effectively. SCR can be used to meet Euro - and even stricter emission standards. Dosing of urea must be controlled to lower NOX emission and NH₃ slip synchronously under the emission standard limits. A type of closed-loop control strategy based on NOX sensors for SCR system was presented in this paper. To detect NOX emissions, two NOX sensors were installed before and after the catalyst. Meanwhile, to examine the trade-off relationship between NOX emission and NH₃ slip, influences of different control parameters to the control purpose were explored. These influences include space velocity, catalyst temperament, NOX conversion efficiency, NH₃ adsorption and desorption characteristics, and so on. Results were used to optimize the dosing control strategy of urea. Base dosage of urea was confirmed based on the signals of NOX sensor.