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In this study, oxalic acid showed better performance than inorganic acid in the indium leaching system from waste liquid crystal displays. The In 3+ leaching efficiency with the powder dosage of 50 g/L came up to 100% within 45 min, using 0.5 M oxalic acid and stewing at 70 °C. Ultrasonic cavitation can improve the powder dosage. Combined with the substance analysis using ICP, XPS, SEM, and XRD, In 2 O 3 dissolution depended on the distributive H + concentration. Metal hydrolysis was a general negative phenomenon for ions loss in an acid leaching system. According to the Arrhenius model, the apparent activation energy for the indium leaching reaction was 43.622 kJ/mol, indicating that the indium leaching rate was mainly affected by the ion diffusion process through the solid−liquid interface and the chemical reaction process. Oxalic acid can maintain an appropriate H + concentration to reduce the acid consumption from nontarget substances to relieve the powder surface corrosion for reuse and to inhibit metal hydrolysis, and its C 2 O 4 2− ions also could precipitate some metal impurities such as Ca 2+ and Sr 2+ in the leachate.