The invention discloses a high-density clarifying tank. The clarifying tank comprises a flocculation reaction device, a clarifying and concentration device which are connected in series, wherein the flocculation reaction device comprises a mechanical agitation zone device, the mechanical agitation zone device comprises an agitation tank and a mechanical flocculation reaction unit arranged in the agitation tank, a hydraulic mixing zone device is connected in series behind the mechanical agitation zone device, a partition wall is arranged between the mechanical agitation zone device and the hydraulic mixing zone device, the bottom of the partition wall is provided with a water communication hole, then the clarifying and concentration device is connected in series with the hydraulic mixing zone device, and a submerged weir is arranged between the clarifying and concentration device and the hydraulic mixing zone device. The invention improves the surface hydraulic load, decreases the occupied area and has the advantages of good flocculation effect, high sludge concentration and good quality of outlet water.