Abstract: The chitosan is used to improve the toughness and the adsorption of heavy metal ions of slag based geopolymer. The effects of chitosan content on the strength, flexural toughness and the adsorption of heavy metal ions of slag based geopolymer were focused, and FTIR, NMR, SEM and EDS was employed to explore the modification mechanisms of chitosan on slag based geopolymer. The results show that the 28 days flexural toughness coefficient of the slag based geopolymer with addition of 2% chitosan increases by 497.22% and its adsorption efficiency of Pb²⁺ and Cr³⁺ increases by 68.89%, 81.45%, respectively, compared with the blank sample. The chitosan can modify microstructures of the slag based geopolymer both in the molecular level, which is the formation of the C-O-Si bond and the reduction of polymerization degree ofSiO₄, and in submicroscopic level, which is the formation of three-dimensional interpenetrating network structure of chitosan and slag based geopolymer. The high adsorption of chitosan and the increase of the C-S-H gel with low Ca/Si ratio are the main reason for markedly enhancement of the adsorption capacity of the slag based geopolymer.