Abstract A new metal–organic nanocomposite with synergistic catalysis function was prepared and developed to construct an electrochemiluminescence (ECL) immunosensor for ultrasensitive detection of tumor biomarker CA125. Silver nanoparticles (AgNPs) and nicotinamide adenine dinucleotide hydride (NADH) that can participate and catalyze the ECL reaction of Ru(bpy)32+ were employed as the metal component and the organic component to synthesize the metal–organic nanocomposite of NADH–AgNPs (NA). The novel ECL immunosensor was assembled via Ru(bpy)32+-doped silica nanoparticles (Ru–SiO2) modified electrode with the NA as immune labels. First, the chitosan-suspended Ru–SiO2 nanoparticles were cast on the gold electrode surface to immobilize the ECL probes of Ru(bpy)32+ and link gold nanoparticles. Then, the primary antibodies were loaded onto the modified electrode via the gold sulfhydryl covalent binding. After immunobinding the analytes of antigen, NA-attached secondary antibodies could be captured as a sandwich type on the electrode. Finally, based on the circularly synergistic catalysis by the silver and NADH for the solid-phase ECL of Ru(bpy)32+, the proposed immunosensor sensed the concentration of antigen. The synergistic ECL catalysis of metal–organic nanocomposite amplified response signal and pushed the detection limit down to 0.03Uml−1, which initiated a new ECL labeling field and has great significance for ECL immunoassays.