A flocculating, grading, and dewatering device includes a tank body, a mixing zone, a flocculent settling zone, a centrifugal dewatering zone, and a particle screening zone. An initial material is mixed with a chemical agent for a flocculation reaction, a lower sediment material settles and an upper liquid overflows. In the centrifugal dewatering zone, a screen basket is allowed to rotate to facilitate a centrifugal movement of the sediment material, water in the sediment material moves into a guide cavity and then flows to a centrifugate outlet, and then a dewatered material in the screen basket is removed out of the screen basket. In the particle screening zone, a screening mechanism screens particles of different size grades in the dewatered material, and water generated flows downward and is discharged; liquids discharged from the centrifugate outlet and the overflow port are combined, and then discharged through a mixed liquid outlet.