Signal amplification for electrochemiluminescence (ECL) has conventionally been achieved by employing effective matrixes that can accelerate the electrochemical redox processes or carry more electrochemiluminophores. Herein, a convenient signal-amplification strategy was proposed for an ECL immunoassay with carboxylated g-C3N4 nanosheets (NSs) as tags and carcinoembryonic antigen (CEA) as the model target via electrochemically pretreating the substrate: a glassy-carbon electrode (GCE) modified with a polymerized 2-aminoterephthalic acid (ATA) film (GCE/ATA). Bioconjugates of g-C3N4 NSs and the signal CEA antibody (Ab2) (i.e., g-C3N4 NS-Ab2) were immobilized on GCE/ATA via a sandwich immunoreaction to form GCE/ATA-Ab1-Ag-Ab2-NSs. Electrochemical-impedance spectroscopy and potential-resolved ECL characterization proved that GCE/ATA plays an important role in the electron-transfer resistance ( Ret) of the GCE/ATA-Ab1-Ag-Ab2-NSs for ECL and that successively scanning GCE/ATA-Ab1-Ag-Ab2-NSs from 0 to -1.6 V in K2S2O8- and H2O2-containing medium could reduce the Ret and bring out 3.3-times-enhanced ECL at the 10th scan cycle compared with that of the 1st scan cycle, which was about 10.2 times the ECL of the GCE/ATA-Ab1-Ag-Ab2-NSs in medium containing merely K2S2O8. Inspired by this, direct and successive scanning of GCE/ATA in K2S2O8- and H2O2-containing medium was employed during fabrication, which dramatically reduced the Ret of GCE/ATA-Ab1-Ag-Ab2-NSs and brought out obviously enhanced ECL responses for selectively determining CEA from 0.1 pg/mL to 1 ng/mL, with a detection limit of 3 fg/mL.