Abstract: For the resource utilization of ferromanganese alloy, a phosphoric acid leaching method was proposed to prepare the precursor solution (manganese-iron phosphate solution) for lithium battery cathode materials. Through single-factor and orthogonal experiments, the optimal conditions were determined as follows: an acid excess coefficient of 1.2, a liquid-to-solid ratio of 11∶1, a temperature of 70 ℃, and a time of 1 h. Under these conditions, the leaching rates of Mn and Fe reached 97.42% and 92.81%, respectively. The impurity content in the purified solution met industrial standards. Thermodynamic analysis confirmed the spontaneous reaction characteristics (ΔG < -40 kJ/mol), and the E-pH diagram of the Mn-Fe-H₃PO₄ system was constructed. It revealed the critical conditions for the stable existence of Mn²⁺ at pH < 2.20 and for the precipitation of Fe³⁺ at pH > 0.56. Kinetic studies indicated that the leaching process followed the unreacted core model, with activation energies of 44.47 kJ/mol for Mn and 48.08 kJ/mol for Fe, suggesting a chemical reaction-controlled mechanism. This work provided a new low-cost approach for the preparation of lithium manganese iron phosphate (LMFP).