<p>This study experimentally investigates Slug flow-induced vibration (SIV) of an S-shaped flexible riser in a small-scale gas-liquid two-phase test loop. The investigation examines slug flow across gas-liquid ratios ranging from 1.0 to 4.0 with a constant superficial mixture velocity. Non-intrusive high-speed cameras simultaneously capture the internal unsteady slug flow characteristics and dynamic responses of the riser. The analysis encompasses slug flow properties, spatial-temporal evolution of vibration displacements, vibration frequency, mode response, and the buoyancy module’s influence on riser response. Results indicate that translational velocity, liquid slug length, and gas plug length increase with rising gas-liquid ratios at constant superficial mixture velocity. The translational velocity shows modest increases, while liquid holdup and slug flow recurrence frequency decrease. Unsteady slug flow induces both in-plane and out-of-plane responses across all experimental cases. At fixed superficial mixture velocity, amplitude increases with higher gas-liquid ratios. The out-of-plane direction exhibits first three order mode responses, while inplane vibration primarily demonstrates fundamental mode response, despite higher-order frequency presence at both ends. The buoyancy module affects both in-plane and out-of-plane responses, with in-plane vibration showing larger amplitude at lower frequency. The buoyancy module’s influence intensifies with increasing gas-liquid ratios.</p>