Abstract: In this study, the micro water solid phase method was used in the carboxymethylation of Gleditsia sinensis Lam. galactomannan (GSG). monochloroacetic acid(MAC) and NaOH were used as the chemical modification reagents to prepare the carboxymethylated G. sinensis galactomannan (CMGSG). The degree of substitution(Ds), Flocculation performance, molecular weight (Mw), viscosity of CMGSG treat with different concentrations of NaOH were investigated. The results showed that when n(GSG):n(NaOH):n(MAC)=1:1:1, the reaction temperature 60℃ and reaction time 12 h,the Ds of the product reached 0.16,with the molecular weight of 1.385×106. CMGSG purified the synthetic wastewater with the minimum turbidity of 20.2 NTU, indicating of a better flocculation performance. The micro water solid phase method had a positive effect on modification of GSG in solvent free system and less reaction reagent.