Using sub-milliFiltration (SMF) module get the sludge of the system divide into high concentration part and low concentration part, in order to make the high concentration part retain activated sludge to strengthen removal rate of organics and the low concentration part meet the need of MBR. Discussing the sludge concentration partition effect under the conditions of small reflux ratio(R 0.5, 1.0, 1.5, 2.0), In addition, investigating the COD removal efficiency of the system with SMF Module, comparing the measured and theoretical value of the MLVSS at each small reflux ratio. Results show that: the method of sludge concentration partition at small reflux ratio condition is feasible, the MLSS of the low concentration part is below 9 g x L(-1) and the high concentration part is above 20 g x L(-1); average removal rate of COD is above 90%; measured value of the two part is related to theoretical value.