Soy protein isolate (SPI) was used as raw material to prepare protein complexes loaded with catechins under different pH conditions. The interaction mechanism was explained by analyzing thermal stability, fluorescence quenching type, binding site number, thermodynamic parameters, and secondary structure content by differential scanning calorimetry, ultraviolet-visible spectroscopy, fluorescence spectroscopy, and circular dichroism spectroscopy. The binding affinity between catechin and SPI, and the emulsibility of the complexes were analyzed. The results showed that fluorescence quenching type of catechin on SPI under different pH was static quenching. Electrostatic interaction was the main force between catechin and SPI when treated under pH 3.5,5.5 and 6.5. Additionally, hydrogen bonds and van der Waals forces were the mainly intermolecular forces for the complex under pH 4.5. Moreover, hydrophobic interactions played major roles in the interaction between catechin and SPI at pH 7.0,7.5,8.5 and 9.5. The thermal stability of the complex gradually increased with the increase of pH value. Furthermore, the SPI denaturation temperature of complex increased to 157.09℃ at pH 9.5. The emulsifying activity and emulsion stability of the complex at pH 7.5 were 7.70% and 13.44% which were significantly higher than those of the control group (SPI) under the same pH (P<0.05). The catechin-SPI complex structure could be changed under different pH treatments. Thus, soybean protein food base with good emulsibility could be prepared by regulating the pH value.