Fluid in rock fractures always continually induces geo-catastrophe in water-rock system engineering. Intuitively observing fluid in fractures is the key method to reveal interaction mechanism of the water-rock under different engineering background, and provide some insights for solving engineering issues. This study proposes the visual method of fluid in rock fractures using enhanced X-ray image digital radiography (EXIDR), and carries out the coupled hydro-mechanical tests on the basis of the material scale of carbonate rocks, red bed mudstone (RBM) and coals. The experimental results show the transition mechanism of pipe flow (PF) to fissure flow (FF) during carbonate rock failures. The flow regime has undergone an evolution process from laminar flow to turbulent flow, also of this change with the fractal characteristics of PF-FF in carbonate rocks under multilevel stress loading. Also, the damage coefficient of RBM under coupled hydrodynamics and multilevel stress loading is non-linearly increasing. Therefore, the initial permeability of RBM under hydrodynamics is significant for geo-hazards prevention in the engineering, which are induced by the seepage and diffusion effects. Besides, the mean square flow (MSF) describes the flow rate varies as the fracture growth and extension, i.e. the fractional exponential evolution law that has a transition changes from super-diffusion flow to sub-diffusion flow. This indicates that fluid in fractures show the double behaviors of anomalous diffusion and nonlinear flow during coal and rock failures.