Cutoff walls have been widely used to prevent seawater intrusion (SWI) in coastal regions. Previous studies generally concluded that the ability of cutoff walls to prevent seawater intrusion depends on the higher flow velocity at the wall opening, which we have shown is not the most critical mechanism. In this work, we implemented numerical simulations to explore the driving force of cutoff walls on the repulsion of SWI in both homogeneous and stratified unconfined aquifers. The results delineated that the inland groundwater level was raised by cutoff walls, which generated a significant groundwater level difference beside two sides of the wall and thus provided a large hydraulic gradient to repel SWI. We further concluded that by increasing inland freshwater influx, the construction of cutoff wall could result in a high inland freshwater hydraulic head and fast freshwater velocity. The high inland freshwater hydraulic head posed a large hydraulic pressure to push the saltwater wedge seawards. Meanwhile, the fast freshwater flow could rapidly carry the salt from the mixing zone to the ocean and induce a narrow mixing zone. This conclusion explained the reason that the cutoff wall can improve the efficiency of SWI prevention through recharging freshwater upstream. With a defined freshwater influx, the mixing zone width and saltwater pollution area mitigated with the increase of the ratio between high and low hydraulic conductivity values (K_H/K_L) of the two layers. This was because the increase of K_H/K_L caused a higher freshwater hydraulic head, a faster freshwater velocity in the high-permeability layer, and the prominent change of flow direction at the interface between the two layers. According to the above findings, we deduced that any way to increase the inland hydraulic head upstream of the wall would improve the efficiency of cutoff walls, such as the freshwater recharge, the air injection, and the subsurface dam.