The P contained in steelmaking slag is regarded as a potential phosphate source, especially with regard to slag with high P₂O₅ content, which is generated from the utilization of high P iron ores. If P can be efficiently extracted from slag, the obtained P can be used as a phosphate fertilizer. Moreover, the remaining slag can be recycled inside the steelmaking process. Compared with other phases, the P-condensed C₂S–C₃P solid solution in slag is more easily dissolved in water; therefore, selective leaching was applied to recover P from slag with high P₂O₅ content. In this study, the effect of K₂O modification on P dissolution in the citric acid solution was investigated, and subsequently, a process for extracting phosphate product from the leachate, via precipitation, was explored. It was determined that K₂O modification promoted dissolution of the solid solution, resulting in a higher dissolution ratio of P. By modification, the majority of the solid solution was dissolved at pH 6, and other phases remained in residue, indicating that a better selective leaching of P occurred. As the pH decreased, the dissolution ratios of both P and Fe increased. Following leaching at pH 5, a residue with a higher Fe₂O₃ content and lower P₂O₅ content was obtained. When the pH of the leachate increased, the dissolved P in the aqueous solution was precipitated. Through separation and calcination, a phosphate product with a P₂O₅ content of 30% was obtained, which has the potential to be used as a phosphate fertilizer.