The invention relates to a method for preparing a micro magnetic carrier, the micro magnetic carrier and an active sludge immobilization method. The method for preparing the micro magnetic carrier comprises the following steps of: adding 10 to 50g/L of Fe3O4 particles into a polyvinyl alcohol solution, stirring, and adding a glutaraldehyde solution, concentrated hydrochloric acid and a sodium alginate solution for reacting; and washing with acetone, ethanol or distilled water, drying, and thus obtaining the micro magnetic carrier using the magnetic Fe3O4 particles as a core for wrapping a polyvinyl alcohol layer. A sequencing batch reactor technology is used in the active sludge immobilization method using the micro magnetic carrier as a core, the inoculation medium consists of the micro magnetic carrier and conventional sewage in a mass ratio of 1:1-1:3 and is used for secondary bio-treatment of active sludge, and the concentration of the inoculated active sludge is 2 to 4g/L (mass of the dry sludge/ effective volume of a reactor); and the operating mode comprises water feeding, anaerobic stirring, aerobic aeration, settlement, drainage and standing. The invention has the advantages of quickening the biochemical reaction rate of dirt degradation and improving the treatment efficiency and the treatment effect of the sewage.