Abstract: The mass transfer rate between steel and slag determines the reaction rates of elements in a molten bath(especially the reaction rates of desulphurization and dephosphorization),and has direct impact on the distribution ratios of elements between steel and slag. In this article,Hangang 250 t converter was taken as the research object. Factors such as gas flow,lance position and bottom blowing location were taken into consideration to study the mass transfer ability of the convertor by a water model whose similarity ratio was 1:9. The results show that increasing the top-blowing intensity and the bottom-blowing intensity can greatly improve the distribution ratios of elements in the molten bath between steel and slag. The mass transfer coefficient between steel and slag decreases obviously with the drop of bottom-blowing air flow,thus a bottom-blowing air flow not lower than 0.06 m³·t^-1·min^-1 is suggested. The more the bottom-blowing holes are,the larger the mass transfer coefficient between steel and slag gets. A distribution plan of bottomblowing air flow is presented,which is superior to the equal distribution plan of air flow.