Activated carbon with high surface area and abundant pore structure is widely used for contaminant adsorption in wastewater treatment. Rice husks (RHs) with unique composition and microstructure are often used as carbon source to prepare activated carbon. Herein, ultra-large specific surface area activated carbon was synthesized from rice husks with pre-activation and activation by supersaturated KOH solution at different temperatures. With the increase of activation temperature, the specific surface area and total pore volume of activated carbon gradually increase. The activated carbon obtained at 900 ℃ shows the largest specific surface area of 3600 m²/g and the maximal total pore volume of 3.164 cm³/g, which are significantly superior to those of the commercial activated carbon (YP-80, the specific surface area of 1310 m²/g and the total pore volume of 0.816 cm³/g, respectively). The highest maximum adsorption capacity for methylene blue was found for the activated carbon with the largest specific surface area, namely, 983 mg/g, which is almost twice as high as that of YP-80 (525 mg/g). By adsorption kinetics fitting, the results are consistent with pseudo-second-order model which indicates that the process of adsorbing methylene blue is chemical adsorption.