In order to address the poor mechanical properties and low water-holding capacity of soy protein isolate (SPI) gels, this study investigated the effects of cold and heat treatments on the gelation properties of mixed solutions of SPI and Premna microphylla Turcz. pectin (PMP) at varying concentrations (0.025%-0.1%, m/m) and their interaction mechanism. The results demonstrated that the water-holding capacity and viscoelasticity of both cold-set and heat-set composite gels were positively correlated with PMP concentration, with hydrogen bonds, electrostatic interactions, hydrophobic force, and disulfide bonds in the gel systems strengthening with increasing PMP concentration. Notably, electrostatic interactions, hydrophobic force, and disulfide bonds played crucial roles in SPI/PMP gel formation, whereas hydrogen bonds contributed minimally. Fluorescence spectroscopy revealed that PMP facilitated SPI unfolding and encapsulated protein molecules, thereby resulting in enhanced fluorescence intensity and reduced surface hydrophobicity. Fourier transform infrared spectroscopy (FTIR) analysis indicated PMP promoted ordered SPI assembly. Scanning electron microscopy (SEM) showed that the pore size of the gels decreased and the lamellar structure became more compact when PMP concentration rose from 0.05% to 0.1%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) confirmed physical binding between SPI and PMP without new compound formation. In summary, the gelation properties of SPI were enhanced by PMP concentration-dependently, but were not markedly affected by thermal treatment. These findings provide a theoretical foundation for the application of PMP in the strengthening of SPI gels and food colloid processing.