To explore the influence mechanism of tides on the intrusion of saline water into adjacent aquifers in estuaries, a two-dimensional transient fully coupled numerical model of river-aquifer parallel to the river was established based on the consideration of the variation of water level and salinity along the river under the action of tides. The flow field and salinity distribution in the river and aquifer under the action of tides were analyzed and compared with those under the condition without tides. The results show that tides promote the rapid intrusion of seawater into the upstream of the river through the estuary and then infiltrate into the adjacent shallow aquifer, forming salt fingers and local circulation, resulting in a large-scale salinization of the adjacent shallow aquifer. Compared with the seawater intrusion in coastal aquifers and the corresponding conditions without tides, the salinization degree and range are significantly increased. Within a single tidal cycle, the degree and range of saline water intrusion in the aquifer fluctuate with the tides, reaching the strongest in the middle of the ebb tide, with a lag compared to the tidal water level changes. Part of the infiltrated salt accumulates in the adjacent shallow aquifer and diffuses to the deep aquifer, and part is discharged to the river with inland groundwater and then flushed to the ocean by inland river water.