Abstract: In navigable areas, underwater oil and gas production systems need to be installed within steel cylindrical protective structures underneath the seabed to prevent damage from activities such as anchoring of passing ships and fisheries. This study aims to reveal the local scouring and siltation characteristics of the seabed beneath the steel cylindrical protective structures and to verify the reliability of the engineering design plans. Based on the steel cylindrical protective structures of the underwater production systems beneath the seabed in navigable areas, experimental research on the scouring and siltation characteristics of the surrounding soil was conducted. Field soil samples were prepared according to the grading data of the engineering area's onsite soil samples, and a sensitivity analysis of the hydrodynamic properties of different backfill soils was carried out to determine the critical initiation flow velocities of different backfill soils, providing references for the selection of backfill materials for the project. Model experiments were also conducted for flat seabed, protruding, and flush-type protective structure types. The results indicate that based on the soil conditions of the engineering site, the amount of scouring and siltation is minimal under various conditions such as the installation of steel cylindrical protective structures and the setting of backfill materials, with no significant scouring and siltation observed. Field measurements have verified the reliability of the experiment results on scouring and siltation of the design plan. The research findings can provide a reference for the design of protective structures underneath the seabed.