The hydraulic conditions of the clearwells in water works are optimized to make the flow as close as possible to the ideal push flow, thus improving the disinfection efficiency and reducing the production of disinfection by-products. In addition to the overall particle outflow that should be studied at T10/t, the velocity, streamline and trace also should be revealed so that the geometry and piston flow could be optimized. Based on full-scale modeling and CFD, the DMP model was loaded to track the particles. The hydraulic characteristics of a clearwell were studied and the regularity of particle movement was revealed. Measures to optimize hydraulic conditions were proposed. The results showed that the completely mixed flow and the piston flow would dominate in different regions. The flow rate in part of the inlet section was about 0.6 m/s, forming a turbulent reflux area. The flow rate at the turn was also slightly larger, which eroded the partition wall and formed a backflow. After the turn, the water flow fully would develop into a piston flow with a flow rate of mostly 0.1–0.2 m/s. At the turn, the residence time of particles at different positions was greatly different, but the outflow could still be concentrated in 3000–5000 s. The perforated wall located at the entrance and the corner replaced by a rounded corner should reduce the backflow and make the water flow more evenly distributed.