To address the complicated groundwater seepage conditions in water-rich sand layer under riverbed influenced by sea tide, the single-borehole dilution method was employed to measure both vertical and horizontal groundwater flow velocities within boreholes, so as to determine the groundwater movement patterns under the fluctuation of river stage. Field test results indicate that the fluctuation of river stage has little impact on the groundwater seepage on both banks, but has great impact on that of alluvial sand bar. If the river stage decreases, the water level of alluvial sand bar will be higher than the river water level, resulting in downward vertical flow in the boreholes. Conversely, when the river stage rises, the water level of alluvial sand bar will be lower, leading to upward vertical flow in the boreholes. The upper silty sand layer of the formation has a certain degree of impermeability, weakening the influence of river water on the deeper strata. As a result, there is no obvious vertical flow in the underlying sand and pebble layer, in which the maximum horizontal flow velocity is as low as 0.48 m/d and will be in favor of the implement of the freezing method.