<p>Acid is commonly used to separate phosphorus-containing solid solutions from steelmaking slag. However, the acid leaching solution obtained from this process cannot be directly utilized and thus requires purification. The effect of different conditions on the calcium and iron removal characteristics of modified steelmaking slag leaching solution was investigated. Additionally, the removal mechanism was analyzed by thermodynamic calculations. The results indicated that the addition of soybean straw ash in steelmaking slag modification enabled K₂O to enter the phosphorus-containing solid solution, promoting phosphorus enrichment. Valuable elements such as phosphorus and potassium were more easily dissolved in the mixed acid. The oxalic acid concentration had a significant effect on the calcium removal rate, whereas the effects of temperature, stirring rate, and time on the calcium removal rate were minor. The main component of the calcium removal precipitate was CaC₂O₄·H₂O, with a removal rate up to 94.48%. During the iron removal process, when the pH value of the solution was low, Fe³⁺ mainly reacted to form the iron hydroxide precipitate for removal. Increasing the pH value of the solution would cause Fe³⁺ to combine with <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\text{H}}_{2}{\text{P}}{\text{O}}_{4}^{-}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mtext>H</mtext> <mn>2</mn> </msub> <msubsup> <mtext>PO</mtext> <mrow> <mn>4</mn> </mrow> <mo>-</mo> </msubsup> </mrow> </math></EquationSource> </InlineEquation>, forming FePO₄·2H₂O precipitate, leading to a reduction in the phosphorus content of the leaching solution.</p>