The clearwell has the dual function of hydraulic regulation and disinfection contact. Improving hydraulic efficiency is an important measure to reduce the amount of disinfection by-products. The example of the clearwell was modeled as a whole. Then the influence of the main geometric dimensions on the hydraulic efficiency of the clearwell were studied by two-dimensional numerical simulation method, and the rationality of the design scheme was verified. The results showed that the length–width ratio had the most obvious influence on hydraulic efficiency, but the trend of t10/T increasing was slowing down. The t10/T value could be increased by reducing the number and width of bends and increasing the diameter of inlet pipe. The Pe under each working condition was greater than one, so the particle translation with the water flow was the main factor of mass transfer. The length–width ratio of the clearwell was 38 and the t10/T was 0.54 which would meet the design requirement of not less than 0.5.