The invention discloses a micron-grade magnetic core coated ferrocyanide adsorbent for removing Cs ions in radioactive wastewater and a preparation method thereof. The adsorbent takes magnetic Fe3O4 as a core, the surface is coated with a dense SiO2 single layer serving as a protective layer, and an active component is metal ion stabilized potassium ferrocyanide coated on the outer layer, wherein stabilized metal ions comprise Ti, Zn, Cu, Ni, Co, and Zr. The particle size of the adsorbent is 0.2-5 μm, the adsorbent in the outermost layer is conductive to improving the adsorption efficiency for Cs+ ions, and an external magnetic field is adopted for realizing solid-liquid phase separation. The preparation method comprises the following steps: coating a hydrated metal oxide of Ti, Zr or Co, Ni, Cu or Zn on the surface of Fe3O4SiO2 to form a composite magnetic material, wherein the hydrated oxide performs hydroxyl polymerization reaction with the surface of SiO2 to produce M—O—Si bonds to improve the bonding strength between M and the surface of SiO2; and finally reacting the composite magnetic material with a potassium ferrocyanide solution to form the required composite adsorbent, wherein the metal ions M achieve the effects of stabilizing the ferrocyanide and also achieve a bridge effect for bonding the ferrocyanide and the composite magnetic material together.