Ultrasound treatment (UST) has been demonstrated to be an effective approach in refining the microstructures of metallic alloys during solidification. The cavitation-induced fragmentation is considered as the major mechanism for grain refinement in the recent study, but the interaction between dynamic bubble motion and dendrite behaviour is rarely investigated previously. In this work, the dynamic behaviour of cavitation bubbles and their interactions with succinonitrile (SCN)-2% (mole fraction) water organic transparent alloy are systematically investigated by high-speed digital image technique and numerical simulation. It is found that increasing the driving pressure transforms the bubble oscillation mode from volume oscillation to splitting oscillation, which significantly enhances the transient pressure and flow strength in the liquid. When a dendrite exists below the bubble, the fracture mode of the secondary branch undergoes a transition from high peripheral fatigue fracture to low peripheral fatigue fracture to overload fracture with the increase of the driving acoustic pressure, and the fracture period decreases with a power function trend. The closer the bubble is to the dendrite, the longeritudinal radius of the bubble in compression is gradually larger than the transverse radius, the bubble shrink time increases, and the minimum bubble volume decreases. In addition, the maximum pressure generated by bubble collapse significantly decreases, and the maximum flow rate first increases and then decreases. When the root radius of the secondary branches decreases or their length increases, the number of fatigue fracture cycles of the secondary branches decreases significantly. The modelling bubble expansion and contraction and secondary dendrite rupture processes are basically consistent with the experimental results, which indicates that the model constructed in this paper can accurately predict the bubble motion and its interaction with dendrites within ultrasonic field.