Abstract: In this study, A micro-water solid phase method was used in the oxidation of Gleditsia sinensis Lam. Galactomannan (GSG) with H2O2 as the oxidizing reagents to prepare the oxidated G. sinensis Lam.Galactomannan(OGSG).The degree of substitution (Ds), relative molar mass and viscosity of OGSG treated with different concentrations of H2O2 were investigated by gel permeation chromatography (GPC), and viscometer detectors. The result showed that with the increasing of concentration of H2O2, molar-mass values and the intrinsic viscosity decreased and degree of substitution and polydispersity increased. With the dosage of GSG and 30% H2O2 20 g and 18 g, the reaction temperature 65 ℃ and reaction time 1 h, the Ds of product reached 1.22.The oxidation of GSG can be achieved by the method of micro-water solid phase oxidation modification under the condition of using fewer reaction reagent.