A filtering assembly for the reverse osmosis water treatment system is disclosed herein. The filtering assembly encloses the membrane unit in an air-tight space formed by a membrane unit cover and a membrane unit housing, both of which could be screwed together and separated apart easily by an ordinary person without using special tools. After the membrane unit is replaced, the original membrane unit cover and housing are re-used to house the new membrane unit. The filtering assembly arranges its tubular connectors for the pipelines at various vertical and horizontal locations so that their connections wouldn't interfere with each other. Additionally, together with the design of separate membrane unit cover and housing, the pipelines could be installed only once and the replacement of the membrane unit requires no disconnecting and re-connecting the pipelines.