Abstract: Under the increasing occurrence of extreme rainstorms, numerous aging check dams in the Loess Hilly Region are facing substantial failure risks due to the absence of spillway facilities and long-term structural aging. However, previous studies have primarily focused on the instability of individual dams, while insufficient attention has been paid to flood cascade propagation mechanisms at the dam-system scale and the storage-capacity trade-off between sediment retention benefits and flood-control safety. Taking the typical parallel dam system in Chelugou Watershed, Xiji County, Ningxia, China, as a case study, this research employed a coupled MIKE11/21 hydrodynamic model to simulate flood propagation and dam-breach evolution under a 300-year return-period extreme rainfall event, analyzing the regulatory mechanism of parallel topology structure on cascade risk. In addition, the Analytic Hierarchy Process (AHP) and a sedimentation prediction model were integrated to evaluate the rationality of dam-system layout and the long-term evolution of flood-control capacity. The results indicate that: ① four key check dams experienced overtopping failures under the 300-year return-period rainfall scenario. Nevertheless, constrained by the combined effects of parallel topology and gully geomorphology, breach floods propagated mainly along individual tributaries without triggering cross-unit cascade amplification. Flood peaks exhibited pronounced temporal staggering, and the maximum inundation depth in the main channel reached 7.25 m. Although local dam failures increased flood hazards within individual sub-units, no basin-wide chain failure occurred, demonstrating the strong risk-blocking capability of the parallel dam system. ② The comprehensive layout evaluation score of the dam system was 0.84, overall at a "reasonable" level. The safety score reached 0.96, whereas the benefit score was only 0.29, indicating that the degree of development and utilization of dam resources is relatively low, and it is still in the stage of lagging behind in the transformation of benefits from "sand interception to land formation". ③ Continuous sediment deposition and reservoir occupation progressively reduce the effective flood-control storage capacity. A significant storage-capacity competition exists between sediment retention and flood regulation functions, which may gradually diminish the long-term flood-safety margin of the dam system.The findings provide scientific support for risk management, storage-capacity regulation, and layout optimization of aging check-dam systems on the Loess Plateau.