The effect of solid bases on de-esterification of soluble soybean polysaccharides(SSPS) was investigated in this paper. A commercial high-methoxy soybean polysaccharide (SSPS) was used as the raw material, and a solid base MgO was used as the catalyst. The effects of reaction temperature, reaction time and catalyst feed ratio on the de-esterification of SSPS were investigated. The de-esterification effects were estimated by the molecular weight, the degree of esterification (DE), and the monosaccharide composition. The results showed that the molecular weight and DE of product decreasedas the reaction temperature increased, while the polydispersity (PDI) gradually increased. When the catalyst amount increased from 1∶100 to 1∶5, DE decreased significantly. However, the changes in the molecular weight and PDI of the product were relatively small. If further increasing the catalyst feed ratio, all indicators changed insignificantly. As the reaction time prolonged, DE of the product further declined and PDI further increased. The degree of esterification decreased more slowly at the laterreaction stage. The results of monosaccharide composition showed that the neutral sugars in side chains were removed most quickly, while the main chain breaks and de-esterification were almost synchronous. Degradation of polyarabinose and polygalactose side chains was slower than that of the main chain. Compared with the bases de-esterification process, the MgO de-esterification process achieved products with higher molecular weight, lower PDI, and better product color if with a similar degree of esterification. The above results indicated that the MgO de-esterification process would have a good application prospect in future SSPS de-esterification.