Soluble soy polysaccharides (SSPS) with different structure characteristics were obtained by controlling the conditions of acid hydrolysis, alkaline de-esterification and proteolysis. The effect of processing on the structure characteristics of SSPS and its foaming properties were discussed in details. The structure properties of SSPS were characterized by monosaccharide composition, average molecular weight, degree of esterification (DE), and protein content. The foaming properties of SSPS were expressed by the foam expansion rate (FE) and the foam stability (FS). The results showed that the acidic hydrolysis process could degrade the SSPS main chain structure and gradually reduce the molecular weight pf SSPS, but could not affect the DE and protein content. With decreasing of molecular weight, the foam expansion rate (FE) of SSPS increased, while the foam stability (FS) decreased. The alkaline de-esterification process did not affect the molecular weight, but reduced the DE and protein content simultaneously. The FE and FS of the product also decreased simultaneously with the decreasing of the DE and protein content. SSPS with different protein content was obtained by proteolysis. Proteolysis did not affect the molecular weight and DE of the product. The results of FE and FS showed the FE and FS of SSPS decreased simultaneously with the decreasing of protein content. All these data suggested that the protein content and average molecular weight of SSPS played a more critical role in its foaming properties compared with the DE. The findings should be of value for the preparation of SPI with good foaming performance.