Abstract: According to the monitoring data of a Akashiwo sanguinea red tide occurred in Xiamen Sea in January and February 2009, the characteristic of red tide, and the relationships of environmental factors and density of Akashiwo sanguinea were analyzed. The results showed that the red tide event is the earliest and longest one in Xiamen sea. Akashiwo sanguinea was most dominant than Skeletonema costatum during the red tide and the process was not disturbed by the changes of water temperature. Higher salinity seems to have triggered the bloom. Besides, the occurrence of the red tide was unchecked by the low concentration phosphate and benefited by the high nitrogen.