Abstract: Flexible risers which are commonly applied to transport submarine resources have been widely utilized in offshore engineering. The intricate Flow-Induced Vibration (FIV) of flexible risers is easily encountered with the presence of both internal and external flows. Once the riser fatigue failure occurs, it causes serious environmental pollution. In this work, a series of tests were conducted in a recirculating water flume to investigate the dynamic behavior of a flexible catenary riser under the combination of the internal liquid water flow and the external shear current. A non-intrusive high-speed imaging technique was employed to record the vibration displacement varying along the riser span. The reduced velocity and the internal flow velocity ranged from 3.55 to 44.69 and from 0.674 m/s to 1.651 m/s, respectively. The influence of the internal flow velocity on the dynamic response was examined. The experimental results including the spatial-temporal response amplitude and frequency as well as the mode evolution of out-of-plane were analyzed. The results show that the response is enhanced with increasing internal flow velocity except for the mode transition cases. In addition, the existence of the internal flow leads to the earlier occurrence of the mode transition. When the lock-in occurs in the vortex-induced vibration of the flexible riser, the amplitude of the flexible riser is magnified with increasing internal velocity, while the new mode response is not triggered. There is a new mode response occurring in the mode transition cases.