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The structural study of lignin-carbohydrate complexes (LCCs) provides a theoretical basis for the high-value utilization of biomass which has always been a hot topic in biomaterials and biomass refining. This paper compares the structure of two types of LCCs in poplar. After poplar wood was crushed by ball milling, water-soluble LCCs and the Björkman LCCs were extracted by hot water extraction and the Björkman classical method, respectively. The molecular weight was determined by GPC, the composition was determined by chemical analysis, and the structure was studied by UV, IR and 2D-NMR. Comparing the two, the results showed that water-soluble LCCs contains a higher content of carbohydrates and has a relatively wide molecular weight distribution, while the carbohydrate content of the Björkman LCCs was relatively low, and the molecular weight distribution range was relatively narrow. UV, IR and 2D-NMR results confirmed that the main chemical links between the lignin units of the two LCCs were β-O-4. β-β, β-5 structures. The chemical linkages between lignin and carbohydrates in water-soluble LCCs were mainly phenyl glycosidic bonds and ester bonds, while in the Björkman LCCs, there were not only have phenyl glycosides bonds and ester bonds, but also found existence of ether bonds between lignin and carbohydrates. This work contributes to a comprehensive understanding of the structure of poplar LCCs and provides theoretical support for the further application of poplar wood.