<p>This study collected 287 samples from nine boreholes in the Sanjiang Plain and analyzed the contents of rare earth elements (REEs), major elements, pH, and organic matter. The study summarizes the vertical distribution characteristics and influencing factors of REEs in the sediments of the Sanjiang Plain. The results show that the vertical distribution of REEs in the sediments of the Sanjiang Plain exhibits a strong grain size effect. Sediments with a particle size &lt; 165.11&#xa0;μm show higher concentrations of Eu depletion, and lower (La/Lu)_NASC and (Nd/Yb)_NASC ratios; while sediments with a particle size &gt; 165.11&#xa0;μm show lower concentrations of Eu enrichment, and higher (La/Lu)_NASC and (Nd/Yb)_NASC ratios. The fundamental reason for this phenomenon is the varying weathering intensities of the sediments. Coarse-grained sediments are less affected by weather, primarily reflecting the REE characteristics of the source area; fine-grained sediments are more strongly affected by weathering, with feldspar weathering producing clay minerals. Meanwhile, primary minerals release Fe²⁺ and Mn²⁺ during weathering, some of which precipitate as Fe and Mn oxides¹, adsorbing REEs and increasing overall concentrations of fine-grained sediments. Additionally, the weathering of feldspar minerals also causes Eu depletion. The REE characteristics of some coarse-grained sediments near the surface are similar to those of fine-grained sediments, likely because human activities increase the organic matter content in surface soil, which adsorbs REEs.</p>