Abstract:  In order to explore the change rule and mechanism of the structural composition and electron transfer capacity of hydrophilic fraction (HyI) during sludge composting. The HyI from sludge composting was investigated in this study by the various analytical techniques, including ultraviolet–visible spectroscopy, excitation–emission matrix spectroscopy, Fourier-transform infrared spectroscopy, two-dimensional correlation spectroscopy, and electrochemical methods, were used to systematically. The spectral analysis revealed that during composting, the contents of aromatic compounds, quinone groups, humic- and fulvic-like substances, as well as aromatization and humification increased in HyI and the content of proteinoid substances decreased. HyI predominantly contained amides and polysaccharides. The content of the carboxyl group in HyI increased, while the content of phenols decreased. Electrochemical analysis showed that the electron-donating capacity of HyI initially increased and subsequently decreased during composting. The electron-accepting capacity first decreased and then increased. Overall, the electron transfer capacities of HyI progressively increased, indicating that the redox capacity of HyI was enhanced during composting. Correlation analysis indicated that the smaller the molecular weight of HyI and the lower the contents of silicate and polysaccharide, the stronger the electron-donating capacity of HyI. Additionally, higher carboxyl group content, greater humic-like substances, higher humification, and higher aromaticity favored electron acquisition by HyI. The results showed that the structural composition and electron transfer capacity of HyI changed significantly during sludge composting. The decrease of HyI molecular weight, the decrease of functional group silicate content and the increase of humification degree were closely related to the electron transfer capacity of HyI. These findings contribute to our understanding of the formation and redox characteristics of HyI during sludge composting, offering insights into its role in environmental pollution control.