Abstract: The effects of surge and sway motion of top platform on the vortex-induced vibration (VIV) of a catenary flexible riser were experimentally investigated based on a circulating water flume. The VIV of the flexible riser possessing an aspect ratio of 125 was tested in an averaged reduced velocity range of 4.40–39.33. The motion of the platform and the vibration displacement of the riser were monitored by the laser sensor and high-speed cameras, respectively. The effects of platform motion on the riser response are examined in terms of the response frequency, amplitude and instantaneous shapes. According to the coincidence of the dominant frequency, the strong and weak coupling between the platform motion and riser vibration are identified. A strong coupling length is proposed to quantify the interaction between them, which is relatively short in the mode transition cases of riser response. According to the in-plane and out-of-plane responses of the flexible riser, there are three coupling patterns with the platform motion, including the bidirectional coupling, unidirectional coupling and uncoupling. Compared to the in-plane response, the out-of-plane response of the riser is less affected by the platform motion. The influences of mode competition and platform motion on the instantaneous vibration shapes of riser are quantified with respect to the number of peaks. The spatial-temporal distribution of response amplitude, vibration shape and dominant frequency before and after the filtering of the natural frequency of platform are compared to figure out the influence. Furthermore, the time weight of strong coupling and the coupling length are proposed to quantify the influence of platform motion on the coupling between the in-pane and out-of-plane responses of the flexible riser. It is found that the platform motion suppresses the spatial mode competition as well as the coupling between the in-plane and out-of-plane responses at high reduced velocity cases.