In response to the problem of water quality purification in rivers and lakes, the water quality purification ability of the water body itself is optimized from the perspective of hydrodynamics. Based on FLUENT software, two-dimensional and three-dimensional hydrodynamic models were established to simulate the changes in the flow field during the water injection process. The impact of different water injection positions on the water quality self purification process was studied using flow velocity, pressure, and turbulence intensity as indicators. The results show that in the depth direction, as the inlet position moves downwards, the active range of the flow field decreases and the dead zone increases. In the flow direction, the farther the flow distance, the greater the turbulence intensity in the flow field. In the width direction, the flow velocity, pressure, and turbulence intensity of the flow field are symmetrical with the longitudinal section of the middle inlet, and the flow velocity is mainly concentrated in the main flow area of the inlet and the reflux at the end. The closer the water inlet is to the upper part, the better the promotion effect of water self purification. It is recommended to use the upper water injection method to promote river self purification. The research results can provide reference and suggestions for promoting the self purification of river water quality.