The extraction of rare earth (RE) elements from ion-adsorption type RE ores relies on in-situ leaching technology. However, the leaching process faces technical challenges such as low leaching efficiency and limited recovery rates. In this study, MgSO₄ was used as the leaching agent, with phthalic acid (PA) introduced as a complexation-assisted additive to improve RE leaching efficiency. The addition of 0.15% PA increased the RE leaching efficiency by 12.4% compared to using MgSO₄ alone. Leaching kinetics results indicate that as PA concentration increases, the RE leaching process exhibits two-stage behavior, with the leaching process governed by the internal diffusion model. Spectrophotometric analysis revealed that the PA primarily forms 1:1 and 1:2 coordination compounds with RE3+, thereby reducing the concentration of free RE³⁺ ions in solution. Quantum chemical calculations further revealed that the order of binding energies for the reactions between the four RE ions and hydrogen phthalate ion (PA-) is: Y³⁺ > Pr³⁺ > Nd³⁺ > La³⁺. The computational results are consistent with the electrostatic potential distribution of RE3+, indicating that PA- interacts more strongly with RE³⁺ ions possessing higher charge density. In aqueous solution, the carboxyl groups of PA release protons, and the resulting oxygen atoms serve as negatively charged sites that coordinate with electron-deficient RE³⁺ ions to form stable RE carboxylate complexes. This interaction promotes the desorption of RE ions from clay mineral surfaces, thereby enhancing RE leaching efficiency.