Abstract: Flow-like motion is the main forms of rock avalanche emplacement, which is the foundation to reveal the dynamic mechanisms of rock avalanches. In this paper, a series of physical modeling experiments based on the Particle Image Velocimetry（PIV）analysis method are conducted to research the internal velocity distribution, internal shear behavior and flow regimes of the granular flow under different grain size conditions. The flow-like motion of rock avalanches and the corresponding deposit features are further discussed and analyzed based on the experimental results. The research results show that the flow-like motion of the granular flow is significantly correlated with the grain size. With the decreasing grain size or the increasing fine particles content, the slip velocity at the bottom of the granular flow and the bulk velocity decrease gradually, the degree of internal shear of the granular flow increases, and the motion form of the granular materials changes from “sliding” to “flowing”. When the grain size is smaller or the content of fine particles is higher, the increasing trend of the internal shear rate is more prominent at the bottom of the granular flow, which indicates that the reduction in grain size promotes shear localization at the bottom of the granular flow. The granular flow regime is different at different locations of the granular flow. The leading and trailing edges of the granular flow are mainly inertial regime, and the interaction between particles is dominated by collisions, while the particle interactions in the main body is dominated by frictional contact. On the surface and bottom of the granular flow, the interaction between particles is mainly collision, while particle interaction in the middle part is mainly frictional contact. For the granular flow with different grain sizes, with the increasing grain size and coarse particles content, the particle collisions in the granular flow are enhanced, and the flow regimes tend to be more inertial.