This paper investigates the characteristics of ammonia storage in actual SCR catalyst, which indicates that space velocities (SV), the exhaust temperature and NH3:NOX ratio (NSR) have different impacts on the saturated ammonia storage and saturated time. Thus, this paper proposes an embeddable SCR model consisting of a nonlinear ammonia storage model (NAS model) and a multiple variable resistance. capacitance model with time-delay (MVRC model), in order to quantify the ammonia storage and system-out NOX concentration. After that, a model-based ammonia storage control strategy was developed to strike a better balance between high NOX reduction efficiency and low ammonia slip.