Abstract: Submarine landslides are common and highly hazardous geohazards in deepwater regions, whose flow behavior is significantly influenced by environmental pressure. However, the underlying dynamical mechanisms under high-pressure conditions lack systematic experimental evidence. To elucidate the controlling effect of deepwater pressure on landslide flow processes, this study conducted underwater granular collapse experiments using a self-developed controllable high-pressure rotating drum system under 0-9 MPa. The landslide dynamic responses were quantitatively analyzed. Results indicate that high-pressure environments markedly reduce the transient fluctuations during collapse, with both the average maximum slope rate of change and the variance of slope rate decreasing significantly as pressure increases, leading to smoother slope evolution. The mean slope rate continuously decreases with increasing pressure, suggesting that high pressure enhances particle-fluid coupling and increases fluid resistance, thereby significantly reducing particle downslope velocity and producing the characteristic "velocity damping effect" of deepwater landslides. Meanwhile, the average flow duration increases significantly with pressure, and the mean flow profile becomes flatter, indicating an overall delayed and prolonged landslide movement, i.e., a "delay effect." This study reveals that environmental pressure systematically regulates landslide dynamics by altering fluid properties and the force balance of particles, providing critical experimental evidence for deepwater landslide hazard prediction, subsea infrastructure safety assessment, and marine geotechnical risk analysis.