Abstract: Flexible risers are widely used in offshore oil and gas transportation. The internal oil-gas mixed flow contributes to the vi? bration response of flexible risers due to the temporal-spatial evolution of fluid pressure and density. Experiments were conducted in an air-water test loop to investigate the vibration response of a flexible catenary riser model excited by internal hydrodynamic slug flows. A non-intrusive technique of high-speed imaging method was employed to record the vibration displacements of riser model and the characteristics of two-phase flow in the riser simultaneously. The gas-liquid ratio ranges from 1.0 to 11.0 with a constant mixture velocity that increases from 0.8 m/s to 3.0 m/s. An in-depth discussion is conducted on the intrinsic link between the spa? tial-temporal evolution of the response displacements， vibration frequency， mode switching and the slug flow features including the length of liquid slug， migration velocity， recurrence frequency of slugs. Experimental results indicate that there is mode switching in time， and three types of mode switching are identified on the basis of the switching characteristics. A partition map of considered cases in terms of mode switching is proposed. The results show that the mode switching mainly depends on the length， recurrence frequency and the distribution of slug flow in the riser model.