In order to improve the emulsification property of soybean whey protein (SWP) and improve the utilization value of soybean byproducts, the complex was prepared from soybean whey protein and dextran sulfate (DS), and its formation mechanism and emulsification characteristics were studied. The ζ-potential, turbidity and spectral methods were used to analyze the structural changes and the interaction mechanism of the protein-polysaccharide complex system. The average particle size and observation of microstructure were used to characterize the distribution and aggregation state of the composite, and the emulsifying characteristics of the composite system were analyzed. The results showed that the pH value and protein/polysaccharide ratio significantly affected the interaction between SWP and DS. SWP and DS could form more stable electrostatic complex through electrostatic interaction at pH value 3.5. Fluorescence spectra and Fourier transform infrared spectra showed that the addition of DS changed the microenvironment and secondary structure of SWP, and the relative content of α-helix in the complex was increased significantly. Hydrogen bond and hydrophobic interaction also affected the formation of the complex. With the increase of DS concentration, the particle size of the complex was increased first and then decreased. When the ratio of SWP/DS was 4, the agglomeration degree of the composite was the highest, and the average particle size was (3667.25±95.32)nm. Under this condition, the composite formed significantly improved the emulsification properties of SWP. The emulsifying activity index and emulsifying stability index were increased by 31.03% and 14.71%, respectively.