Abstract: The mechanism of ultrasound promoting soy protein isolate (SPI) -saccharide graft reaction was studied by using soybean protein isolates as the research object.Compared with classical wet-heating, content of free amino acid increased from 0.86mol/L to 0.98mol/L when SPI was treated by ultrasonic for 60min.Surface hydrophobicity and content of apparent sulfhydryl content were also increased to a certain extent respectively.In this case, polypeptide chains of protein were more stretch, and part of free amino groups buried in the molecules inside were exposed.At the same time, α-helix and β-folding of SPI was significantly reduced while irregular coil was significantly increased, that showed ordered molecule structures become disordered and its steric hindrance reduced.The graft reaction of SPI-saccharide was accelerated because their more contacting led by intense vibration of ultrasound.