The location of membrane units in the membrane tank is a key factor in the construction of a full-scale membrane bioreactor (MBR), as it would greatly affect the hydrodynamics in the tank, which could in turn affect the membrane fouling rate while running. Yet, in most cases, these units were empirically installed in tanks, no theory guides were currently available for the design of a proper location. In this study, the hydrodynamics in the membrane tank of a full-scale MBR was simulated using computational fluid dynamics (CFD). Five indexes (i(Lu), i(La), i(Lb), i(Lint), i(Lw)) were used to indicate the unit location, and each of them was discussed for their individual impact on the risk water velocity (v(0.05)) in the membrane unit region. An optimal design with all the indexes equaling 0.6 was proposed, and was found to have a promotion of 146.9% for v(0.05).