A device for treating harmful algal blooms using modified clay includes a power system, a screw conveying storage bin storing and conveying modified clay to a mixed flow system, the mixed flow system that continuously disperses the modified clay in water which is pumped in in site and endowed an impulse so that an adjustable turbulence is formed to instantly adopt the fed powder and then make sure the slurry uniform and ripened, a high-pressure sprinkling system for pressurizing a ripened modified clay slurry and sprinkling the slurry onto the surface of the bloomed water, and a central control system that controls the whole auxiliary device.