In order to reveal the hydraulic reasons for the uneven distribution of water flow and velocity in the high-efficiency clarification tank, and the hydraulic reasons for the deposition of sediment on one side, CFD was used to simulate the overall tank. The three-dimensional simulation results showed that without any baffle, the velocity was extremely uneven, and the rotation of the stirring blade around the axis produced a velocity loop, which resulted in a large backflow of the velocity. There were obvious differences in flocs and water flow through the two holes. After adding the existing baffles, the water flow and floc concentration could be much more uniform, but there was still sludge sediment on one side. After further lengthening the existing baffles outside the cylinder and adding 45° baffles in the transition section of the coagulation tank and the sedimentation tank, the inconsistency of flow velocity and floc concentration could be basically solved.