Abstract: In order to clarify the interaction between soybean protein isolate (SPI) and κ-carrageenan (κ-CG) under different pH and blending ratio conditions, the phase behavior, structural characteristics and binding mechanism of SPI/κ-CG composite system were characterized by turbidity, zeta potential, multi-spectral analysis and fluorescence quenching methods. The results showed that as the pH decreased (from 8.0 to 1.5), the SPI/κ-CG composite solution exhibited different phase behaviors such as clarification, turbidity and phase separation. The critical pH (pH_c, pH_φ1, pH_opt and pH_φ2) decreased with the decrease of protein/polysaccharide mass ratio. The apparent viscosity of SPI/κ-CG composite solution was the highest at pH4.0 when the interaction between the two was the strongest. The addition of κ-CG changed the tertiary structure of SPI, and the content of β-sheet in the secondary structure increased first and then decreased with the decrease of pH, while the content of β-turn decreased first and then increased. In addition, fluorescence quenching proved that κ-CG had a static quenching effect on SPI, and the two had strong binding ability. Thermodynamic analysis showed that the interaction between SPI and κ-CG was dominated by hydrophobic interaction and electrostatic interaction, which was a spontaneous reaction driven by enthalpy change. This study revealed the interaction mechanism between SPI and κ-CG, and provided a reference for the development and application of SPI/κ-CG complexes in the field of food.