With the aim to reveal the interaction between soybean protein isolates (SPI) and dextran (Dex) in the mixed system and the effect of dextran concentration (1%, 3%, 5% and 7%) on the structure and functional properties of SPI, fluorescence spectrum, ultraviolet spectrum and Fourier transform infrared spectrum were used to characterize the conformational changes of the SPI-Dex non-covalent polymer, and the effect of dextran concentration on the functional properties of SPI was analyzed through particle size, surface hydrophobicity, turbidity, solubility, emulsification activity, emulsification stability and antioxidant activity. The results showed that SPI and Dex can interact through two noncovalent forces: hydrophobic interaction and hydrogen bond under neutral conditions, thereby changed the structure and functional properties of SPI. The addition of Dex can prevent the exposure of tryptophan and tyrosine residues, and form a tighter tertiary structure compared with SPI alone. When the addition of Dex in the mixed system was less than 5%, with the increase of Dex, the particle size, surface hydrophobicity, and turbidity of SPI-Dex polymer were decreased significantly, and the solubility, emulsification, and antioxidant activity of SPI-Dex polymer were improved significantly. When the concentration of Dex in the mixed system was 5%, the improvement effect on the functional properties of SPI was the most significant, the solubility, emulsifying activity index and antioxidant activity of SPI were increased by 16.35%, 18.71% and 11.30%, respectively.