Weathered crust elution-deposited rare earth ores (WREOs) serve as a major source of medium and heavy rare earth elements(REEs). Currently, in-situ leaching with ammonium sulfate is widely used for REEs extraction from WREOs. However, this method is often limited by low leaching efficiency and slow leaching kinetics as well as poor mass transfer. In this study, humic acid (HA) was employed to improve the leaching performance of rare earth elements during the in-situ leaching of WREOs. Under optimized conditions at HA concentration of 0.2 g/L, pH 6.5, and leaching temperature of 298 K, a maximum rare earth leaching efficiency of 95.27% was achieved. This enhancement was attributed to an improved mass transfer process in the presence of HA, as explained by chromatographic plate theory. The mass transfer rate was further found to increase under moderately elevated temperatures (303-323 K) and in a weakly acidic environment (pH 4-6). Additionally, the presence of HA was shown to reduce adhesion work and surface tension, both of which contributed to an improved mass transfer rate of rare earths. Furthermore, the leaching kinetics of rare earths in the presence of HA were systematically examined, leading to the development of a new kinetic equation that accurately describes the in-situ leaching process. Analysis based on this equation confirmed that the leaching process was controlled by internal diffusion.