[Objective] Frequent urban waterlogging caused by insufficient flow capacity of stormwater drainage systems under extreme weather conditions has become a pressing issue. This study aims to investigate the variation in flow capacity of a stormwater drainage system with rectangular box culverts，considering the geometric parameters of an L-shaped sediment barrier at the inlet and the backwater effect （downstream water-level-induced backwater）. [Methods] A dual-method approach was adopted，combining hydraulic model tests with 3D computational fluid dynamics （CFD） simulations. The hydraulic model，based on a gravity-driven similarity criterion （λ_L=20），simulated a typical stormwater drainage system in Eastern China consisting of rectangular box culverts and multiple manholes. Flow rates were measured using standard thin-walled triangular weirs，and downstream water levels were precisely regulated to simulate backwater conditions. Hydraulic model tests were conducted to analyze the flow capacity of stormwater drainage system inlet structures under conditions with and without L-shaped sediment barriers and backwater effects. Following the hydraulic model tests，numerical simulations were conducted using ANSYS Fluent. The SST k-ω model was selected，and the numerical model was validated against experimental data，with deviations in total head and piezometric head kept within 10%. To quantify the impact of the L-shaped sediment barrier，three primary geometric dimensions were systematically varied： the opening size in the flow direction （a），the opening size perpendicular to the flow direction （c），and the vertical height of the barrier （e）. [Results] Sediment barriers and water level differences significantly dictate the system’s drainage efficiency. Hydraulic model tests showed that removing the sediment barrier entirely could increase flow capacity by up to 64.69% compared to the test with the barrier in place. The numerical analysis of the barrier’s geometric parameters provided deeper insights into structural optimization. Among the studied variables，the dimension perpendicular to the flow direction （c） was found to be the most influential factor，resulting in a 7.68% increase in flow rate. In contrast，the dimension in the flow direction （a） showed a moderate impact，resulting in a 4.98% improvement in drainage efficiency. The vertical height （e） proved to be the least sensitive parameter，with reducing it from 650 mm to 350 mm yielding only a 2.27% gain in capacity. Furthermore，the study highlighted a strong positive correlation between the inlet-outlet water level differential and the overall flow rate. When the downstream backwater effect was minimized by lowering the water level to 86.00 m，the flow capacity showed a dramatic increase even without any structural changes. Crucially，the benefits of geometric optimization became more pronounced under low water levels. Specifically，with the high head difference provided by the 86.00 m downstream level，optimizing the perpendicular dimension （c） yielded a 17.54% improvement rate，significantly outperforming the gains observed at higher downstream levels. [Conclusions] While structural optimization of inlet components is beneficial，addressing downstream backwater effects in the primary lever for enhancing stormwater drainage performance.Among the geometric parameters of L-shaped sediment barriers，widening the opening perpendicular to the flow direction is identified as the most effective structural modification for inlets. Accordingly，a dual-stage strategy combining water level regulation and structural optimization is proposed.The study provides a scientific basis for urban waterlogging mitigation under extreme rainfall scenarios and offers novel insights into the optimized design of stormwater drainage systems，demonstrating substantial engineering application value.