The use of open-type check dams in mountainous areas has become common practice in order to mitigate the effects of debris flow and extend the service life of engineering structures. The beam dam, a common debris flow control system, has received less attention in research on the impact process of debris flow and check dams compared to solid check dams. Additionally, the estimation of impact pressure in debris flow primarily considers debris flow characteristics, without taking into account the influence of geometric characteristics of the transmission structure. To better understand the impact process of debris flow on beam dams, a series of small-scale debris flow impact tests were conducted in a model flume. Key parameters, including velocity, depth, and impact pressure, were measured. The results show that the maximum impact pressure of debris flow is affected by both the characteristics of the debris flow and the relative opening size of the beam dam. Due to flow and edge occlusion in the middle of the beam dam, the discharge of debris flow is enhanced, resulting in a longer impact process and higher maximum impact pressure. Based on these findings, a calculation model of the maximum impact pressure of debris flow at the midpoint of the middle beam is proposed, which can be used to estimate the impact of debris flow on the discharge part of the beam dam.