Abstract: This paper, based on the refractive index matching technique, conducted a detailed study on the dynamic spreading process of dense granular columns during the immersed granular collapse over a rough bed. It explores the effects of liquid phase viscosity and the aspect ratio on the characteristics of the collapse and presents the development and motion process of granular flow at typical cross-sectional positions. The results indicate that the rough bed exerts a restraining effect on the transport of the granular, which is specifically manifested by a reduction in the dam's runout length and an increase in the final height. Besides, the energy transfer efficiency from the granular to the liquid on the rough bed is decreased, significantly weakening the final accumulated wave distribution. Additionally, in some working conditions, the phenomenon of the granular accumulation body's front edge lifting is observed, but its physical mechanism remains to be elucidated.