Ammonium sulfate in situ leaching process is the most widely used extraction technology for ionic rare earth (RE) ores. Although the leaching process is simple, the leaching rate is satisfactory; moreover, the leaching solution has high impurities, serious ammonia pollution, and other problems. The leaching rate and leaching efficiency using a single ammonia-free leaching agent are low, particularly for low-grade complex ores. In this study, a leaching experiment and kinetic analysis of a combined leaching agent for an ionic RE ore with low grade and high impurity content were carried out. The results show that the leaching rate of RE3+ is 97.58%, and the concentration of Al3+ in the leaching solution is 0.016 g/L when a combined leaching agent composed of QZX-02 and ammonium sulfate (mass ratio of 7:3) was used to leach the ionic RE ore. Compared with using ammonium sulfate alone, the leaching rate of RE increased by 8.96%, and the concentration of Al3+ decreased by 0.069 g/L. Both of the leaching process of ammonium sulfate and the combined leaching agent are controlled by internal diffusion kinetics, and the apparent activation energies are 8.59 kJ/mol and 7.34 kJ/mol, respectively. The lower apparent activation energy makes the combined leaching agent possess a higher leaching efficiency. The analysis of Fourier transform infrared (FTIR) spectras and scanning electron microscopy (SEM) images indicate that the combined leaching agent and impurity ions could produce carboxylic acid organic compounds on the surface of clay minerals, with the result that the leaching of impurity ions is suppressed and the leaching efficiency of RE ions is improved. The application of this combined leaching agent can alleviate the ammonia pollution caused by the ammonium sulfate leaching process and effectively improve the utilization rate of RE resources. (C) 2017 Elsevier B.V. All rights reserved.