<p>The use of marginal soils with high fines content as backfill in geosynthetic-reinforced soil (GRS) slopes can lead to the development of pore water pressures (PWPs) and intensification of seepage forces under variable water levels, posing stability challenges. Current design guidelines provided limited consideration to hydraulic effects. This study proposed a modified top-down procedure based on the limit equilibrium method to evaluate the internal stability of GRS slopes under variable water pressures. The results were validated through comparisons with existing analytical, numerical, and centrifuge test results. Parametric studies were performed to investigate the impacts of water level variations, reinforcement length, and reinforcement layout on slope stability. Results indicated that PWPs significantly increase reinforcement tension. A critical reinforcement length incorporating PWP effects was proposed to resist both pullout and compound failures. A “sparse-dense” (SD) reinforcement layout, characterized by fewer reinforcements in the top reinforced zone and a greater number in the bottom reinforced zone, effectively reduced the required reinforcement strength and length under adverse effects of PWPs. Rapid external water drawdown induced a sudden increase in tensile forces in zones affected by the water level decline, increasing the risk of localized failure.</p>