Abstract: Skeletonema costatum is a widely distributed species in global waters, which also causes red tide frequently. It is of great significance to detect and monitor S. costatum for marine ecological environment research. In this study, a rapid visual in-situ detection method for S. costatum was developed based on rapid nucleic acid extraction and loop-mediated isothermal amplification. This method offers the advantages of high specificity, high sensitivity (1.2 pg genomic DNA) and rapid detection time (<100 min), and the presence or absence of cells can be determined through color changes in the reaction system. The method has been verified by a large number of field samples and holds great feasibility, which is expected to be widely used in field monitoring of S. costatum.