Abstract: In order to explore the change rules and mechanisms of the structural composition and electron transfer capacity of the hydrophilic fraction (HyI) during sludge composting, the HyI was investigated in this study using various analytical techniques, including ultraviolet-visible spectroscopy, excitation-emission matrix spectroscopy, Fourier-transform infrared spectroscopy, two-dimensional correlation spectroscopy, and electrochemical methods. The spectral analysis revealed that during composting process, 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 process. The electron acceptance capacity first decreased and then increased. Overall, the electron transfer capacities of HyI gradually increased, indicating that the redox capacity of HyI increased during composting process. 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, more humic substances, higher humification, and higher aromaticity were beneficial to the electron acquisition of HyI. The results showed that the structural composition and electron transfer capacity of HyI changed significantly during sludge composting process. The decrease in HyI molecular weight, the decrease in functional group silicate content, and the increase in humification degree were closely related to the electron transfer capacity of HyI. These findings contribute to our understanding of the formation and redox properties of HyI during sludge composting process and provide insights into its role in environmental pollution control.