Abstract: Soy protein isolate (SPI) is widely favored in the food industry due to its high nutritional value and low cost. However, its inferior foaming performance inherently limits its large-scale industrial application. In this study, the effects of composite polysaccharides (Lonicera japonica polysaccharide and chitosan) on the foaming properties and structural characteristics of SPI were investigated. Multiple characterization techniques, including Fourier transform infrared (FT-IR) spectroscopy, fluorescence spectroscopy, circular dichroism (CD) spectroscopy, ultraviolet absorption spectroscopy, were applied to clarify the structural change and the interaction between SPI and composite polysaccharides. The results demonstrated that the incorporation of the composite polysaccharides significantly improved the foaming capacity and foam stability of SPI. The optimal mass ratio of Lonicera japonica polysaccharide to chitosan was 3∶1, and the optimal mass ratio of the composite polysaccharides to SPI was determined as 1∶10. Mechanistic analysis revealed that SPI interacted with composite polysaccharides mainly via hydrogen bonding and electrostatic attraction, which promoted the formation of larger molecular aggregates. Compared with pure SPI, the surface hydrophobicity of modified SPI decreased by 17.29%-22.27%, which was attributed to the coverage of hydrophobic groups induced by polysaccharide binding. Moreover, structure characterization showed that the α-helix content of SPI secondary structure decreased, whereas the content of β-sheet increased. In addition, both the particle size and absolute Zeta potential value of SPI were elevated, facilitating the construction of a stable colloidal system. The composite polysaccharide induced structural rearrangement of SPI molecules, strengthened intermolecular interactions, and stabilized the air-water interface, thereby effectively improving the foaming performance of SPI. In summary, the improved foaming properties of SPI were primarily ascribed to polysaccharide-induced structure modification and enhanced surface activity. This study provides a theoretical basis and technical reference for the high-efficiency application of SPI in the food industry.