The invention provides a method for treatment of oily sewage at the bottom of a bilge of a vessel and a method for preparation of magnetic activated carbon for decontamination of the oily sewage. The method for treatment of oily sewage at the bottom of the bilge of the vessel includes steps of feeding magnetic activated carbon into a mixing tank, filtering and retaining suspended particles in sewage at the bottom of the bilge and the magnetic powdered activated carbon with partial oil adsorbed by means of a high-gradient magnetic filter, feeding magnetic powdered activated carbon into an ultra-filtration membrane reaction tank, and deoiling by means of the magnetic powdered activated carbon and an ultra-filtration membrane. The method for preparation of magnetic activated carbon includes the steps of soaking powdered activated carbon into clean water for more than twenty-four hours, taking out the powdered activated carbon and drying the powdered activated carbon for two hours at a temperature higher than 105 DEG C, ultrasonically dispersing the dried powdered activated carbon with ferroferric oxide at the ambient temperature, and filtering by means of a filter screen and drying the filtered activated carbon for more than two hours at a temperature higher than 105 DEG C. More loosened mud cake layers are formed on the surface of the ultra-filtration membrane, filterability is greatly improved, membrane flux is improved, and membrane pollution can be effectively prevented.