Abstract: The slag entrapment process and the flow field of a specific ladle at critical flow rate are investigated based on particle image velocimetry(PIV) technology,and the change of fluid velocity at the region where slag is entrapped into steel during the whole slag entrapment process is studied by water modelling. The influence of the kinematic viscosity of slag on the critical slag entrapment velocity is analyzed quantitatively. The critical slag entrapment velocity collected from experiments is compared with the calculated one by the traditional theory. It is shown that the critical slag entrapment process can be divided into 8 stages from the occurrence to the end,and the fluid velocity at the point where slag is entrapped into steel increases first,then decreases and increases again during the 8 stages. The kinematic viscosity of slag has great influence on slag entrapment. When the kinematic viscosity of slag increases,the critical flow rate and critical slag entrapment velocity enlarge,and the linear correlation relationship between the critical slag entrapment velocity and the kinematic viscosity of slag becomes more obvious. A relational expression between the critical slag entrapment velocity and the kinematic viscosity of slag is derived from experimental data by liner fitting. At the end of this article,a correctional expression of the critical slag entrapment velocity is put forward based on the experimental data and the theoretically calculated values.