Developing an electrochemiluminescence (ECL) coreactant to minimize the biotoxicity and to maximize the enhancement factor is critical to single-cell ECL microscopy. Here, we reported a guanine-rich single-stranded DNA (G-ssDNA)-loaded high-index faceted gold nanoflower (Hi-AuNF) as a synergistic coreactant of Ru(bpy)32+ for single-cell ECL imaging. Because of the excellent catalytic performance and huge specific surface area, Hi-AuNF serves as not only an ECL enhancer but also a carrier for G-ssDNA. Guanine in G-ssDNA specifically reacts with Ru(bpy)32+ through a so-called "catalytic route" and thus significantly enhances the ECL signal of Ru(bpy)32+. To endow targeting ability to the synergistic coreactant, an aptamer of carcinoembryonic antigen (CEA) is incorporated into the G-ssDNA to form G-ssDNA-Apt for the recognition of human breast adenocarcinoma cells, which overexpress CEA on the cytomembrane. Accordingly, the ECL imaging of CEA on the cytomembrane was realized by using the highly selective Hi-AuNF@G-ssDNA-Apt as the probe as well as the luminophore of Ru(bpy)32+. Compared with the common coreactant tripropylamine with high toxicity and volatility, the Hi-AuNF@G-ssDNA-Apt is considered as a high-performance and biocompatible coreactant, providing exciting opportunities in single-cell imaging and detection.