Sludge age (SRT) is one of the primary influencing factors in the system of short-cut nitrification, in order to select a suitable SRT, in the stable operation of SBR, with the simulated wastewater as treatment object, the response effect and the sludge floc characteristics of short-cut nitrification are investigated by changing the SRT. Experiment has proved that the longer SRT the more complete nitrification. When the SRT for the 10d time, the NO2--N accumulation rate was maximum. When the SRT is short, the biological floc phase composed mainly by bacilli, and the surface of the sludge floc is rough and structure is loose. When the SRT is longer, the reactor's biological floc phase mainly composed by the coccus and has got relatively smooth surface with a compact structure. Rod-shaped bacteria are the good characteristics of the phenomenon of the short-cut nitrification.