The invention relates to a magnetic bead for efficiently adsorbing extracellular DNA in an environment, and a preparation method thereof. The magnetic bead is of a nano spherical double-layer structure, the particle size is 15-30 nm, the specific surface area is 65-210 m<2>/g, the inner layer is magnetic nano ferroferric oxide (nFe3O4), the outer layer is a mesoporous-loaded silane coating layer, and the magnetic bead contains hydroxyl functional groups. Firstly, nano ferroferric oxide (nFe3O4) magnetic beads are wrapped with silicon dioxide, then a coating layer is formed again by using a surfactant, the magnetic beads capable of efficiently adsorbing extracellular DNA in the environment are obtained after the surfactant is eluted, and the magnetic beads have the characteristics of small particle size and large specific surface area, can highly selectively adsorb extracellular DNA in the environment in the presence of pollutants, and can be used for efficiently adsorbing the extracellular DNA in the environment. The adsorption time is short, the adsorption capacity is large, and the method can be used for efficiently extracting extracellular DNA in the environment.