This study, an outdoor flume model test, was to model the movement of different-diameter boulders under the complete fluid-solid coupling with different-density landslides, analyze the movement patterns of boulders in landslides, the factors influencing the movement of boulders, and the force condition of boulders in a landslide and also explore the effect of the debris flow density and boulder diameters on the velocity of the boulders in landslides. The study found that the movement of boulders may be rolling, leaping or sliding, which is related to their density and diameter. That is, the greater the boulder’s diameter and the smaller its velocity, the simpler its movement pattern in the landslide, it is more likely to roll, the better its following behaviour in the landslide; also, it is more likely for the speed ratio (n) to approach 1. The study showed that the boulder diameter affects boulder velocity more greatly than the landslide density.