Realtime and online detection of reverse osmosis (RO) membrane pollution is a key problem, but few researches was done in this area home and abroad. In this paper, the research of measuring the fluidflow of RO membrane by using RTD is outlined, and the diagnose system of spiralwound reverse osmosis is introduced, and data acquisition, display and processing of RTD response signal for RO membrane module are completed using LabVIEW software, and detection experiments and data analysis under the same condition of new and old membrane are done. The results showed that this method could realize realtime online diagnosis of RO system and be widely applied to RO seawater desalination projects.