Abstract: The microorganisms can clearly affect the mobility of metal ions in the environmental fluids because of abundant chemical groups on the cell surface. Adsorption of ions on the microorganism surface plays important roles in various biogeochemical processes, such as mineralization, nucleation and crystal growth. In order to disclose the adsorption mechanisms and to understand the adsorption capacity of bacteria, this paper experimentally studies the Cu2+ adsorption behavior of a widely occurring bacterium, Paenibacillus polymyxa. Firstly, consecutive acid titration was carried out. The results show that the range of optimum pH at which the Paenibacillus polymyxa absorbs H+ is 7.54 ~ 6.00, and the surface of Paenibacillus polymyxa cell bears abundant negative charges, which determines the ion adsorption capacity. Then, the Cu2+ adsorption isotherm of Paenibacillus polymyxa was measured. As the olution pH increases linearly with the increasing Cu adsorption capacity, it is deduced that the ion exchange between surface H+ and Cu2+ may be the potential mechanism of the apparent adsorption. The data fitting indicates that the biosorption process agrees well with both the Langmuir model and Freundlich model. According to the isotherm equation of Langmuir model, the calculated adsorption capacity of Cu2+ is as high as 1.69×10-7 mg/cell.