In order to study the variation characteristics of the flocculation in radial-flow secondary sedimentation tank, the Ghadiri model was used in this research to simulate the crushing mechanism of floccules. The core of aggregating functions was defined by UDF. The RNG k-ε turbulence model, Mixture multi-phase model and Aggregation model, and the Aggregation model which considered the velocity difference, Brownian motion and Turbulence intensity were used in this study. Considering flocculation , the three-dimensional numerical simulation was carried out under different inflow velocities and inflow temperatures in the radial-flow secondary sedimentation tank. The results showed that, the effects of flocculation on the sedimentation efficiency of the radial-flow secondary sedimentation tank was significant. The high temperature inflow was more effective for the growth of the sludge floccules. The sedimentation efficiency of the sedimentation tank was improved when the inflow velocity were selected between 0.02 m/s and 0.05m/s.