Defatted soybean meal was employed as an original material to get β-conglycinin (7S) through separation and purification. And 7S was grafted with maltodextrin by using ultrasound-assisted to modify its properties. Taking the emulsifying activity (EAI) and emulsion stability (ESI) of the modified products as indexes, Box-Behnken model was used to optimize reaction conditions. In addition, SDS-PAGE electrophoresis and scanning electron microscope (SEM) analysis were also investigated to verify that 7S molecule was grafted with the saccharide molecule. Response surface analysis revealed that the optimized modified conditions were as follows: the amount of maltodextrin of 9.5%, ultrasound time of 30min , ultrasound power of 450W. On these conditions, the emulsifying activity index (EAI) and emulsion stability index (ESI) reached to 1.142 and 27.13min, which were 60.59% and 57.06% times against the native β-conglycinin, respectively.