The invention provides a method for preparing sludge-based formed magnetic active carbon. The method comprises the steps as follows: sludge is used as a raw material, an activating agent is added for carbonizing and activating the sludge; nano-sized Fe3O4 is used as a magnetic source for magnetizing the sludge active carbon; and sodium carboxymethylcellulose and aluminum salt are composited to an organic-inorganic binding agent, the binding agent and a p-toluenesulfonhydrazide foaming agent are used for uniformly binding powdery magnetic active carbon into a formed magnetic active carbon precursor with abundant pore structures, and finally, the formed magnetic active carbon precursor is heated to obtain the sludge-based formed magnetic active carbon. According to the method, the sludge is used as a main raw material, the resource utilization of the sludge is achieved, the recycling is convenient, and reutilization can be achieved; and further, the sludge-based formed magnetic active carbon has a stable structure and good magnetic stability, and can be widely used in the fields such as sewage treatment, soil pollution treatment and the like.