Abstract: The spatial distribution characteristics of chlorophyll-a (Chla) in phytoplankton in the Xiangxi River during the flood season and their relevant affecting factors were analyzed by applying mathematical statistics methods and based on the monitoring data of hydrodynamics, meteorology and the drifting of the algae in Xiangxi River reservoir of the Three Gorges Reservoir during the flood season (August 4-10, 2017). The process of horizontal transport and vertical mixing of algae under the background of stratified density flow was explored. The results showed that: 1) The hydrodynamic data of Xiangxi River reservoir was featured with the obvious density-stratified flow. Reynolds numbers during the monitoring period were all above 4 000, and the water body got into the turbulent state, which produced two effects on the movement of the algae, i.e. horizontal movement and vertical mixing. 2) The horizontal drift velocities of the algae on the water surface was affected by wind speeds and water velocities with the response formula V_algae=0.035V_wind+0.461V_water+0.034 (R²=0.917, P<0.01). 3) The dilution effect of density-stratified flow on the raw water of high-concentration of algal cells in the reservoir and the carrying effect of circulations on high-concentration algal cells led to a linear downward trend of the total Chla concentrations of the water column. Meanwhile, the mainstream poured back into the reservoir at the upper and middle layers, and the upstream inflow flowed to the river mouth in the form of density current at the bottom along the slopes. The two flows met in the middle and upper sections of the river and thus a circulation was formed, and the pouring-back flow was divided into two parts in the middle of the reservoir, which were respectively discharged from the surface and the bottom. Chla concentrations presented such a sequence: surface > bottom > middle. When the circulation was formed as a result of the clash between the flow poured back and the raw water of the reservoir and discharged from the bottom to the reservoir bay, Chla concentrations presented this sequence: surface > middle > bottom, or middle > bottom > surface. The results suggested that the wind speeds and water horizontal velocities of density flow were key factors for the horizontal distribution of Chla, while the vertical distribution was affected by the water circulation mode formed by density flow.