The invention discloses a method and system for preparing activated carbon. The method comprises (1) crushing phyllostachys heterocycla to obtain crushed phyllostachys heterocycla materials; (2) feeding the crushed phyllostachys heterocycla materials to a revolving bed pyrolyzing furnace for pyrolysis to obtain pyrolytic oil and gas and pyrolytic bamboo charcoal; (3) feeding the pyrolytic oil and gas to a gas-liquid separation device for gas-liquid separation to obtain pyrolytic gas and a mixed liquid of tar and bamboo vinegar; (4) supplying a part of the pyrolytic gas to a gas-fired boiler as a fuel gas to obtain a high-temperature steam; and (5) supplying the pyrolytic bamboo charcoal and the high-temperature steam to a rotary activating furnace to activate the pyrolytic bamboo charcoal, so as to obtain the activated carbon and a high-temperature flue gas. The method can process the pyrolytic bamboo charcoal obtained through pyrolysis of phyllostachys heterocycla into the high value-added activated carbon without an external energy source, is low-carbon and friendly to environment, and is especially suitable for large-scale promotion.