Abstract: The treatment of car-washing wastewater by using membrane bioreactor (MBR) was investigated. The influences of operating pressure and environmental factors on the membrane's filtration function were analyzed combining with theoretical calculations. In the meiobar range, membrane resistance was the dominant role, independent of operating terms. In the middle pressure range, concentration difference-polarized resistance was the dominant role, depending much on operating terms. In the high pressure range, gelatin resistance was the dominant role. The influences of the operating pressure was small on the membrane flux, and the mem-brane's separating characteristics were related much with mud concentration, membrane flow velocity and etc. This study showed that when MBR was used to dispose car-washing wastewater, there was a critical pressure in MBR. When the operating pressure was higher than the critical pressure, the membrane flux was changed slightly, but membrane surface pollution was increased distinctly. Therefore when MBR was designed, the operating pressure was a key parameter. The influences of environmental factors such as temperature, pH and hydraulic retention time on the membrane's filtration function were investigated.