The invention discloses a method for extracting genomic DNA. A cell lysate (5-50mmol/L Tris.Cl(pH8.0), 0.2-20mmol/L EDTA(pH8.0),0.1-10 percent SDS) is added into a sample, and a sample cell is lysed; RNase digestion is performed; the sample is cooled, a protein precipitation liquid (1-10mol/L (NH4)2SO4 or 1-10mol/L NH4Ac) is added, and protein is removed through centrifugal precipitation; and a supernatant fluid after the centrifugation is transferred into 100 percent isopropanol, and the DNA is centrifugally precipitated. Compared with the prior art, the method can quickly obtain high-purity high-quality genomic DNA, the whole process only needs 30 minutes, and an obtained DNA fragment is about between 50 and 500kb. And the method has the advantages of simple operation, no need of complicated equipment, low cost, cleanliness, no toxicity, and no harm to users and the environment.