Circulating microRNAs (miRNAs) play a pivotal role in the occurrence and development of acute myocardial infarction (AMI), and precise detection of them holds significant clinical implications. The development of luminol-based luminophores in the field of electrochemiluminescence (ECL) for miRNA detection has been significant, while their effectiveness is hindered by the instability of co-reactant hydrogen peroxide (H₂O₂). In this work, an iron single-atom catalyst (Fe-PNC) was employed for catalyzing the luminol-O₂ ECL system to achieve ultra-sensitive detection of myocardial miRNA. Target miRNA triggers a hybridization chain reaction (HCR), resulting in the generation of a DNA product featuring multiple sticky ends that facilitate the attachment of Fe-PNC probes to the electrode surface. The Fe-PNC catalyst exhibits high promise and efficiency for the oxygen reduction reaction (ORR) in electrochemical energy conversion systems. The resulting ECL biosensor allowed ultrasensitive detection of myocardial miRNA with a low detection limit of 0.42 fM and a wide linear range from 1 fM to 1.0 nM. Additionally, it demonstrates exceptional performance when evaluated using serum samples collected from patients with AMI. This work expands the application of single-atom catalysis in ECL sensing and introduces novel perspectives for utilizing ECL in disease diagnosis.