The invention discloses a device and a method for removing algal bloom and purifying an algae source in a branch reservoir bay of a channel reservoir. The device comprises an algal bloom removing unit, an algae source purifying unit and a clarifying unit. The feature of periodic immersion and exposure of a hydro-fluctuation belt of the branch reservoir bay of the channel reservoir is ingeniously utilized, the device is built according to the geographic features of the hydro-fluctuation belt and is quite convenient to install and maintain at the low water level; the algal bloom removing unit can rapidly layer, locate and eliminate the algal bloom in a water body during the high-water-level algal bloom; on one side, the algae source purifying unit controls the flow state of the water body based on the fluid mechanics principle to assist in enhancing the implementation effect of the algal bloom removing unit, and on the other side, the algae source purifying unit is used to destroy the biological activity of algae-source tiny cells in the water to eradicate in depth the algae-source tiny cells in the water; and the clarifying unit adopts a rubber dam, so that the main natural function of the reservoir is not affected during the period without algal bloom.