Since the Euro-ІІІ standard was adopted, the main methods to inhibit NOx production in diesel engines are exhaust gas recirculation (EGR) and select catalyst reduction (SCR). On these methods SCR offers great fuel economy, so it has received wide attention. However, there also exists a trade-off law between NOx conversion efficiency and NH₃ slip under dynamic conditions. To inhibit NH₃ slip with high NOx conversion efficiency, a dynamic control method for a urea water solution (UWS) injection was investigated. The variation phenomena of SCR conversion efficiency with respect to the cross-sensitivity characteristics of the NOx sensor to NH₃ have been thoroughly analyzed. The methodology of “uncertain conversion efficiency curve tangent analysis” has been applied to estimate the concentration of the slipped NH₃. The correction factor “φ” of UWS injection is obtained by a comparative calculation of the NOx conversion ability and subsequent NH₃ slip. It also includes methods of flow compensation and flow reduction. The proposed control method has been authenticated under dynamic conditions. In low frequency dynamic experiments, this control method has accurately justified the NH₃ slip process and inhibits the NH₃ emission to a lower level thereby improving the conversion efficiency to a value closer to the target value. The results of European transient cycle (ETC) experiments indicate that NH₃ emissions are reduced by 90.8% and the emission level of NOx is close to the Euro-Ѵ standard.