Abstract: Marine risers are usually hinged under a platform, experiencing the Vortex-Induced Vibration (VIV) due to the excitation of wave and current and hence the potential fatigue damage. In this paper, the non-intrusive optical test technique was employed to monitor the vibration displacement of a catenary flexible riser arranged in a water-circulating flume. The aspect ratio of the top-hinged and bottom-fixed riser model is 125. It was found that the in- and out-of-plane excited modal order and frequency grows with the reduced velocity. The maximum root-mean-squared amplitude in the out-of-plane direction underwent distinct undulation during the mode transition, while it was not obvious in the in-plane response. Nevertheless, the dominant frequency has a significant rise when the mode transition occurred. The mode transition was out of sync in the three directions. In mode transition cases, the dominant frequency and mode varied along the riser length, suggesting the spatial mode competition. The profile of root-mean-squared amplitude depended on the spatial distribution of mode weight and the dominated riser length by the dominant mode. Comprehensively taking into account the number of the peaks of root-mean-squared amplitude profile and the consistency of vibration frequency, four interaction patterns are proposed, including the strong coupling⁺, strong coupling, weak coupling and weak coupling⁻. Accordingly, the space partition of the response interaction in the considered reduced velocity range is provided.