Herein, Zn2+-induced gold cluster aggregation (Zn2+-GCA) as a high-efficiency electrochemiluminescence (ECL) emitter is first employed to construct an ECL biosensor to ultrasensitively detect microRNA-21 (miRNA-21). Impressively, Zn2+ not only can induce the aggregation of monodispersed gold clusters (Au NCs) to limit the ligand vibration of Au NCs for improving ECL emission but also can be utilized as a coreaction accelerator to catalyze the dissociation of coreactant S2O82- into sulfate radicals (SO4•-) to improve the interaction efficiency between Zn2+-GCA and S2O82-, resulting in further intense ECL emission. Compared to Au NCs stabilized by bovine serum albumin with ECL efficiency of 0.40%, Zn2+-GCA possessed high ECL efficiency of 10.54%, regarding the [Ru(bpy)3]2+/S2O82- system as a standard. Furthermore, output DNA modified with poly adenine (polyA) obtained from enzyme-free target recycling amplification can be efficiently immobilized on the surface of gold nanoparticles (Au NPs) to reduce the defect of special design, cumbersome operation, and low stability. Thus, an ultrasensitive ECL biosensor based on the Zn2+-GCA/S2O82- ECL system and enzyme-free target recycling amplification achieved ultrasensitive detection of miRNA-21 with the detection limit of 44.7 aM. This strategy presents a new idea to design highly efficient ECL emitters, which is expected to be used in the field of bioanalysis for clinical diagnosis.