A novel and simple method was explored to obtain (polyacrylic acid) PAA-g-MWCNTs by grafting acrylic acid (AA) onto the surface of the multi-walled carbon nanotubes (MWCNTs) using azobisisobutyronitrile (AIBN) as an initiator. The results of TEM, FTIR, XPS, Raman spectra and TG proved that AA is successfully grafted onto the surface of MWCNTs. Contact angle measurement and dispersion tests revealed that the hydrophilicity of PAA-g-MWCNTs is significantly higher than that of pretreated MWCNTs. The adsorption experiments of methylene blue (MB) showed that the adsorption capacity is proportional to the degree of functionalization and increases with initial concentration, pH and temperature. The maximum adsorption capacity is about 329.8 mg g(-1). The kinetic and isothermal studies also showed that the adsorption data are consistent with pseudo-first-order and pseudo-second-order kinetic and Langmuir isothermal models. The remarkable adsorption capacity of PAA-g-MWCNTs can be attributed to the interaction of various adsorption mechanisms, and further analysis indicated that hydrogen bonding and electrostatic attraction play a decisive role in the adsorption process. In addition, the relationship between oxygen-containing groups content (OC) and MB adsorption capacity was revealed by a simulation using a mathematical model.