The effects of particle size, temperature, and leaching time on the leaching behavior of rare-earth elements were studied. The leaching efficiency of the rare earth reached 39.24% under leaching conditions of hydrochloric acid concentration of 8.00 mol/L, particle size 95% distributed below 1.5 μm, leaching time of 120 min, and temperature of 90 °C. According to the scanning electron microscopy and X-ray diffraction analysis of the residue, the mechanism of the leaching process was also investigated. Actually, the phase of REFCO transformed into that of RECl and REF but there was an existing intermediate transition, where the phase of REFCO on reacting with hydrochloric acid generated that of REO F and this process also released RE into the solution. REO F continued reacting with hydrochloric acid to release a lot of F, which on combining with RE formed REF precipitation. The leaching kinetics of rare earths follows a shrinking core model that can be expressed as 1 - 3(1 - ) + 2(1 - ) = . The activation energies are 62.1, 54.8, 35.1, and 34.9 kJ/mol, respectively.