<p>Sediment trapping is a critical function of check dams. Affected by climate change and earthquakes, the increased loose deposits in gullies have led to frequent outburst debris flows, and many check dams face challenges related to sediment accumulation. Bed slope is a key factor influencing sediment trapping, but the impact of the bed slope on the sediment trapping effect of open—type check dams has not been clearly elucidated. Therefore, to investigate the operational status of check dams in the Longnan area in China, indoor flume tests were carried out for six bed slopes (specifically 5°, 6°, 7°, 9°, 10°, 12°) to simulate the interaction between open-type check dams and outburst debris flows. In particular, the effect of changes in the values of a range of parameters of the sediment behind the dam (pore-water pressure, total mass, particle size, and deposition morphology) was studied. Drawing on the well-established theoretical model, a preliminary analysis was conducted to determine the physical mechanisms causing these changes. Results show that as bed slope increases, pore water pressure behind the open-type check dam rises, sediment mass behind the dam decreases quadratically, and sediment trapping efficiency declines. When slope exceeds a certain value (&gt; 10° in this study), debris flow sediment transport capacity increases. This causes initially deposited particles behind the dam to be re-entrained and continue moving downstream, significantly increasing the siltation slope. This, in turn, affects the particle size of the deposited material behind the dam. Furthermore, theoretical analysis indicates that the impact force on an open-type check dam and the particle size of the sediment behind the dam are directly proportional to bed slope. These findings can guide the design of debris flow check dams in mountainous regions and inform local government maintenance policies.</p>