Selective withdrawal has been widely used in managing reservoir water quality. However, theoretical studies have been confined to using a single linear distribution of the water density and have not addressed other circumstances in detail. In this paper, the thickness of a selective withdrawal layer with a bilinear density distribution was determined using the conventional analysis method, and the coefficient C-3 was obtained. By the software, FLUENT, the vertical domain of selective withdrawal using a bilinear density, the coefficient C-3 and the relevant velocity distribution were obtained. The results show that the withdrawal thickness was similar to the corresponding theoretical value. The velocity analysis results were also in accord with the results of corresponding experiment. However, there is a significant difference in the location of the maximum velocity between the single linear and the bilinear density distributions, with the former located on the orifice height, while the latter not being so.