SCR after-treatment is the mainstream technology to reduce the NO x emission amount. The self-developed SCR system’s composition, operating and reactor mechanism were presented, and the coordinate control strategy was used to assure that the system’s inner module was operated regularly, the original emission model and chemical reaction kinetics model were adopted to obtain the raw value of urea supply amount, the exhaust temperature correction model was introduced to fix the instantaneous emission temperature, the relation between exhaust temperature and amount of NH 3 storage was used to effectively control the NH 3 leak amount, and failure warning was made according to the OBD regulation when NO x emission exceeded the standard. Each operation point had a NH 3 coverage ratio threshold which can ensure NH 3 leakage volume ratio was less than 1.0×10 -5 during the rapid heating process and try to improve the NO x conversion efficiency. According to the research results of the NH 3 coverage ratio and storage, the estimation model was established, and the leakage control strategy was put forward. The sensors and actuators of the system, original machine emission MAP and target conversion rate MAP were calibrated. The dosing control unit using above control strategy was adopted in the target engine. Test results showed that the brake specific emissions of NO x in ESC and ETC were 1.52 g/(kW ·h) and 2.09 g/(kW ·h), respectively, which indicated that the self-developed SCR control system can effectively reduce the NO x emission amount and reach the national Ⅳ emission regulation.