In water-rich strata, a traditional vertical barrier exhibits certain limitations when applied to deep foundation pit construction under complex geological conditions, such as it is difficult to completely cut off deep and thick aquifer, which may pose potential risks during pit dewatering. To address the above challenge, this study introduced a mixed barrier system in which the horizontal barrier (HB) was set at the bottom of the foundation pit and was combined with the enclosure wall to collectively retard groundwater seepage into the pit. Based on an actual project in Tianjin, this study established HB models with varying numbers of its layers using ABAQUS 6.14 software. It systematically investigated the effect of HB on groundwater drawdown, ground surface settlement, and enclosure deflection during foundation pit dewatering. The research shows that HB can significantly reduce the magnitude of external water level drawdown by altering groundwater seepage paths while effectively controlling soil settlement. Furthermore, it exhibits favorable overall restraining effects on wall deformation. Varying the number of horizontal barrier layers (L) exhibits an insignificant effect on water-blocking and subsidence-control performance. However, the constraint effect on the enclosure shows a correlation with L.