Ultrasensitive detection of nucleic acid has attracted considerable attention recently in academic research and clinic diagnostics. Current approaches for DNA analysis involve complicated or expensive processes for labeling and often yield a high detection limit. In this study, a hydrogel electrode prepared from graphene oxide and fish sperm DNA is used for label-free mitochondrial DNA detection by impedimetric approach. The hydrogel has a bionic structure containing rich water and natural biomolecule fish sperm DNA that would benefit the adsorption and hybridization of DNA. Graphene oxide is a semiconductor and its conductivity can be improved by doping negatively charged DNA molecules. The result shows that the conductivity and impedance change of hydrogel electrode could be tuned by its length and component. The linear range for DNA detection by the optimized hydrogel is from 1.0 x 10(-9) to 1.0 x 10(-20) M with a detection limit of 1.0 x 10(-20) M. The result is ascribed to the bionic structure and tunable conductivity of hydrogel electrode. The hydrogel electrode has been used to detect the real DNA samples from patients of ovarian cancer with satisfactory results.