The invention provides a method for preparing active carbon by utilizing urban sludge. The method comprises the following steps: a) standing by, drying, crushing and grinding the urban sludge, and then calcining under a vacuum condition; b) removing inorganic substance from the matter acquired in the step a); c) regulating pH value of a solution obtained in the step b) to be alkaline, washing till being neutral, and then washing with absolute ethyl alcohol; d) baking and grinding the matter obtained in the step c), thereby obtaining the active carbon. According to the method provided by the invention, the urban sludge is taken as the raw material and then is successively processed according to the steps of standing by, drying, crushing, grinding, vacuum calcining, removing inorganic substance, regulating pH value to be alkaline, washing till being neutral, and then washing with absolute ethyl alcohol, baking and grinding, so that the sludge-base activated carbon with larger specific area can be acquired. The active carbon is a mesoporous material, can be used as an electrode material and also can be used as an absorbing material.