Abstract: On the condition that the oil-water phase contact area was kept constant at room temperature, a model experiment was done on a 3-D tubular reactor using diesel engine oil and water to simulate the slag and steel, respectively, thus investigating the mass transfer phenomena by taking the advantage of sodium benzoate as a tracer that is soluble in both water and nonvolatile oil. Changing the circulating water flow rate uninterruptedly, the mass concentration of sodium benzoate in water was measured by the PH-3G conductivity meter to calculate the mass transfer rate. Based on the double membrane theory, the restrictions on mass transfer of sodium benzoate in the oil-water two phase boundary layer were judged with the equilibrium distribution ratio calculated. Furthermore, the diffusion coefficient of sodium benzoate in water was obtained on the basis of binary liquid kinetics theory, and the dimensionless number equation to determine the Reynolds, Sherwood and Schmidt numbers were given under the experimental conditions.