Abstract: Stratified density currents have been existed in Xiangxi Bay for a long time. This study simulates the characteristics of these currents for the period of 2012-2014 using the hydrodynamic and water quality model CE-QUAL-W2, and analyzes their variations, the cause and the risk of algal blooms in the bay through combining the simulations with the data of river runoff and water temperature collected from the section between the Yangtze mainstream and the Xiangxi tributary in flood season. The results show that in the three years of 2012-2014, the depth of the stratified density currents intruding into the bay from the mainstream were varying in the period from the middle to the end of July or the early August, in a typical cycle from middle-upper depth to middle-lower depth and then back to middle-upper depth. The main cause was the increasing mainstream runoff that brought about a change in the trend of mainstream water temperature and then led to a change in the water density difference between the mainstream and the bay. The observed currents evolved lagging behind the increasing of the mainstream flow. A certain variation in water temperature in a corresponding time period is required before it can play a significant role in the formation or evolution of the currents. Generally, the evolution should lower the risk of algal blooms in Xiangxi Bay in flood season.