The process of pollutant mixing is significantly influenced by secondary flow and turbulence in meandering rivers. To investigate the influence of different point source release positions on the pollutant mixing process in sinuous open channel flows, a 3D large-eddy simulation (LES) model based on OpenFOAM was established to simulate the process of passive scalar transport in a sinuous channel with a rectangular cross-section. After verification by a flume experiment, two sets of cases in which the point sources were arranged at identical intervals in spanwise and streamwise directions were configured to evaluate the mixing efficiency. The effect of flow structure, secondary motion, and the turbulent viscosity on the scalar transport and mixing was discussed. The distribution of scalar as well as the scalar flux was analyzed in detail, and the fluctuation characteristics were also described. The results demonstrate that due to the existence of secondary flow in the sinuous channel, different transverse and streamwise release positions of the point source have significant influence on mixing efficiency and spatial distribution of the pollutant. The point source placed near the center of the cross-section in transverse or near the apex of the bend in streamwise result in higher mixing efficiency. Mixing efficiency calculated by different indices can be different, which requires comprehensive assessment.