The work aims to discuss the structural changes of lignin in the process of preparing the molding products with the high yield pulp (HYP) as the raw material, provide theoretical basis for the study on the strength formation mechanism of the molded products prepared by the raw material with high content of lignin represented by HYP, and lay a foundation for gradually forming the theory of formation of fiber bonding strength based on the pulp molding technology. By means of gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), two-dimensional heteronuclear signal quantum coherence (2D-HSQC) spectra and quantitative 31P-NMR, the comparison of the lignin structures before and after the pulp molding hot pressing and drying process was simulated. The influence of hot pressing process on the lignin structure was studied. Based on the hot pressing process with the dahurian larch HYP as raw material, the molecular weight of MWL in the molding pulp boards (MPB) increased from 5216 g/mol to 6132 g/mol. FT-IR analysis confirmed that the non-conjugated carbonyl groups of MPB-MWL decreased while the conjugated carbonyl groups increased. 2D-HSQC analysis confirmed that the resinol (B) structure percentage from HYP-MWL to MPB-MWL decreased from 8.66% to 6.68% (with aromatic ring as the internal standard, the same below), while that of the phenylcoumarans (C) structure increased from 15.45% to 20.88%. 31P-NMR quantitative results showed that, the structures of aliphatic hydroxyl groups, G type phenolic hydroxyl groups, and p-coumarates in MPB-MWL were lower than those in HYP-MWL. The increase in molecular weight of MPB-MWL illustrates that the polymerization of lignin occurs in the process of hot pressing. FT-IR analysis indicates that both the cleavage of non-conjugated (C=O) chemical bonds and the formation of con-jugated (C=O) chemical bonds coexist during the hot pressing. 2D-HSQC and 31P-NMR analysis confirms that depolymerization and polymerization occur simultaneously, which are a pair of competitive reactions, but the extent of polymerization is greater than depolymerization.