Electrochemiluminescence (ECL), an emission of light excited by electrochemical reactions, has drawn attention as tools across diverse fields, ranging from clinical disease diagnosis to photo-/electro-catalysts development. Nonetheless, the ECL efficiency of most luminophores in aqueous solutions is low, which significantly hamper their broad applications; thus, understanding the intrinsic factors for ECL efficiency is highly envisioned. Herein, taking emerging carbon nitride (CN) with unique electronic structures and rigid 2D backbone as a model luminophore, we report that the orbital delocalization was a promising unifying factor for its ECL efficiency. Behind the complicated transformation of molecular structures regarding cyano-terminal groups and triazine/heptazine basal frameworks, the orbital delocalization of the as-prepared CN was found to be generally improved at an elevated condensation temperature. Such intrinsic evolution in electronic structure favored the electron injection in excitation and photon emission afterward in ECL of CN. As a result, the cathodic ECL efficiency of CN was remarkably improved to a new milestone of 170-fold greater than benchmark Ru(bpy)3Cl2.