[Objective] The disaster reduction effects of the debris-flow check dam on downstream structures under varying rainfall frequencies were investigated to offer guidance for disaster prevention and mitigation in villages and towns. [Methods] First， a field investigation of the ‘7 · 12’ Heishui gully debris-flow event in Pingwu County， Sichuan Province， China， was conducted. Second， based on the ultimate bearing capacity of masonry walls under debris flow impact， critical velocity formulas for bending failure and shear failure of brick masonry walls were proposed using static equilibrium methods. Finally， the debris-flow movement process and deposition extent in Heishui gully， both with and without the check dam and under varying rainfall frequencies， were simulated using FLO-2 D. [Results] ① The scale of debris-flow discharge， sediment deposition depth and flow velocity were intricately linked to the frequency of rainfall， with a simulation accuracy of 85% in comparison to the field investigation data. ② The wall suffered bending damage due to the overall pressure of debris-flow slurry and shear damage from the impact of large boulders. ③ In the absence of a check dam and at a rainfall frequency of 1%， the debris-flow deposition area was 9.36×10⁴ m²， the maximum flow velocity in the construction zone reached 7.19 m/s， and the maximum debris-flow depth in the construction zone was 5.25 m. At a rainfall frequency of 2%， the debris-flow deposition area was 5.63×10⁴ m²， the maximum flow velocity in the construction zone reached 5.63 m/s， and the maximum mud depth in the construction zone was 4.46 m. ④ After the check dam was constructed， the debris flow did not reach the outlet under the 2% rainfall frequency， resulting in an 82.6% reduction in the deposition area. Under the 1% rainfall frequency， the debris-flow deposition area and maximum flow velocity in the construction zone were diminished by 38.4% and 60.6%， respectively. [Conclusion] Prevention and control measures can significantly reduce the magnitude of disasters and prevent buildings from being damaged. However， monitoring and early warning systems require enhancement throughout the rainy season.