<p>Boulders within debris flows pose significant risks to infrastructure and safety due to their mass and impact force. This study explores how boulder size, shape, and debris flow density influence boulder motion through flume model experiments. We found that boulder centroid velocity correlates with debris flow surface velocity, which is the primary factor driving boulder velocity fluctuations. Boulder motion is divided into two stages: initiation, dominated by sliding, and steady motion, characterized by a combination of rolling and sliding. Larger boulders exhibit lower centroid velocities and weaker coupling with the debris flow. While boulder shape significantly affects motion mode, with spherical boulders primarily rolling and others sliding or rolling, it does not notably impact the velocity ratio between the boulder and the debris flow. Debris flow density directly influences the degree of coupling, with higher densities leading to a more synchronized velocity ratio. These findings provide insights into boulder dynamics within debris flows, offering valuable guidance for disaster prevention and mitigation in mountainous regions.</p>