Electrochemically generated chemiluminescence (ECL) has attracted significant interest over decades, ranging from fundamental studies of highly efficient electron-tophoton interconversion to practical bioassays. Nonetheless, the ECL efficiency of most emitters is low, which greatly hampers further development. Herein, we report a highly robust carbon nitride film with an unusually boosted ECL efficiency (2256 times higher than that of the reference Ru(bpy)3Cl2/K2S2O8. Double inoculation, which provided the primary interaction of carbon nitride with the substrate and the succedent growth, played a crucial role in preparation. The improved ECL efficiency was ascribed to few pinholes suppressing futile co-reagent reduction, maintenance of more orbitdelocalized heptazine subunit improving ECL kinetics, and transparency avoiding selfabsorption. As a result of the exceptionally high ECL efficiency, an ultrasensitive visual DNA biosensor by the naked eye was further successfully developed.