This paper deals with the effect of the pH value on the Zeta potentials,ultracentrifugation combined with electrophoresis,solubilities,fluorescence spectra and secondary structures of soybean protein isolates,with the mechanism of the subunit dissociation of the soybean protein isolates in acid conditions being also analyzed. The results show that the subunit dissociation is mainly due to the intermolecular electrostatic repulsion,and the extent of the dissociation of glycinin is much greater than that of β-conglycinin,that the Zeta potential reaches the maximum at a pH value between 2. 0 and 3. 0,and that the subunits dissociate when the pH value is 2. 0 and 3. 0. Moreover,it is found that,at pH 3. 0,the extent of the dissociation reaches the maximum,the average diameter of the soybean protein isolates is 2. 1 times that at pH 7. 0,the random coil content of the secondary structure increases by 15. 7%,and the maximum absorption wavelength in the fluorescence spectra increases by 4. 3 nm,and that,the extent of the dissociation decreases when the pH value is lower than 3. 0,the subunits do not dissociate when the pH value is 1. 0 and 7. 0,and the solubility of the undissociated protein decreases dramatically with the increase of the rotating speed of the ultracentrifugation.