Abstract: This paper summarizes the basic principles of residence time distribution (RTD) technique and its applications in membrane separations. It introduces in detail the research processes of RTD application in microfiltration，nanofiltration and reverse osmosis. The most important research field is spiral wound reverse osmosis membrane module. It also introduces RTD research progresses in characterizing flow patterns of membrane modules and optimizing their performance. The paper demonstrates fields in which RTD technology can be applied. For example，the membrane companies can optimize membrane module design; membrane engineering companies can select the best membrane modules; membrane customers can detect and diagnose the fact of fouling and scaling within membrane modules. Meanwhile，it presents the future research and development of RTD technology as well.