A water model experiment was used to investigate the migration behavior of nonmetallic inclusions at the steel–slag interface in the steelmaking process. Based on the two basic principles of static similarity and dynamic viscosity similarity, an experimental model was constructed with alumina and polypropylene particles representing inclusions, water as the simulation fluid of liquid steel, and silicone oil as the simulation fluid of steel slag. On the basis of this model, a pump with adjustable power was added to simulate the paralleling flowing liquid steel. By adjusting the viscosity of silicone oil, particle material, diameter and water flow speed, the movement mechanism of particles floating into the water–oil interface was systematically observed and recorded, and the mechanism of different factors affecting the migration behavior of inclusions was analyzed. It is found that the particles will encounter the deformation resistance of the interface and be bounced downward when they reach the water–oil interface, and then float up for a second time and may successfully cross the water–oil interface. In addition, this study also found that the particle type, size and interface wettability are key factors affecting particle movement at the interface. Due to their high wettability, polypropylene particles show larger dimensionless displacement than Al₂O₃ particles under the same oil layer conditions. At the same time, the increase in particle size helps them to cross the water–oil interface, and the increase in oil layer viscosity increases the resistance of particles to cross the interface. In addition, it was found that the change in water velocity in the dynamic water model also has an effect on the motion of particles, and the increase in flow velocity helps to reduce the difficulty of particles passing through the water–oil interface.