Apply the slag ion theory to calculate the distribution ratio formula of phosphorus in the smelting reduction process of high-phosphorus iron ores, according to which it is found that main factors affecting the distribution ratio of phosphorus are ferrous oxide activity, phosphorus capacity of slag, and phosphorus activity coefficient in molten iron. Through thermodynamic theory and experimental research on dephosphorization, it comes out that the distribution ratio of phosphorus increases with the increase of ferrous oxide activity in slag, the linear regression slope is 1.65, approximate to 2.5 in the classic formula. With the increase of basicity, the phosphorus capacity of slag would increase. Both dissolved oxygen and dissolved carbon in molten iron can increase the activity coefficient of phosphorus in molten iron. It would have beneficial to the distribution and the transfer of phosphorus to slag. Both the weighted basicity of slag (B) and the distribution ratio of phosphorus (Lp) can accurately describe the distribution ratio of phosphorus in the smelting reduction process of high-phosphorus iron ores. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.