The invention relates to a wet desulphurization device and a dust-fall processing method for circulation sizing based on the wet desulphurization device and relates to a dust-fall processing method and a dust-fall desulphurization system. The sizing which is rich in dust in a circulating oxidation pool in an absorber is partially extracted and pumped into a gypsum hydrocyclone; the bottom flow inthe gypsum hydrocyclone which is rich in gypsum returns to the absorber; the liquid and solid separation is partially performed on the overflow which is rich in smoke; the separated solid matter which is rich in dust is discharged; the separated clear liquid is pumped back to the absorber; a stirrer is used for stirring the sizing in the circulating oxidation pool; a sizing outlet on a desulphurization tower main body is connected to an inlet end of a gypsum sizing pump; an outlet end of the gypsum sizing pump is connected to an inlet end of the hydrocyclone; an overflow port of the hydrocyclone is connected to an inlet of a liquid-solid separating device; and the bottom outlet of the hydrocyclone is connected to the desulphurization tower main body. The method and device provided by the invention can be used for the wet desulphurization device which is rich in smoke and fuel low sulfur coal smoke and also can be used for the wet desulphurization device for further increasing the quality of desulphurization by-product.