We herein describe a solvothermal method for preparing superparamagnetic Fe₃O₄ magnetic colloidal nanocrystal clusters with an average diameter of 195 nm. The Fe₃O₄ magnetic colloidal nanocrystal clusters were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) and other methods. The Fe₃O₄ magnetic colloidal nanocrystal clusters were used to extract genomic DNA from fresh blood, Gram-negative bacteria, Gram-positive bacteria and frozen blood. Results showed that the genomic DNA integrity, purity and yield extracted via this universal method were satisfactory. This method is rapid, simple and does not use protease K or toxic compounds. This method lays the foundation for future research and nucleic acid extraction techniques.