The accumulation of dysfunctional giant mitochondria is a hallmark of aged cardiomyocytes. This study investigated the core mechanism underlying this phenomenon, focusing on the disruption of mitochondrial lipid metabolism and its effects on mitochondrial dynamics and autophagy, using both naturally aging mouse models and etoposide-induced cellular senescence models. In aged cardiomyocytes, a reduction in endoplasmic reticulum-mitochondrial (ER-Mito) contacts impairs lipid transport and leads to insufficient synthesis of mitochondrial phosphatidylethanolamine (PE). A deficiency in phosphatidylserine decarboxylase (PISD) further hinders the conversion of phosphatidylserine to PE within mitochondria, exacerbating the deficit of PE production. This PE shortage disrupts autophagosomal membrane formation, leading to impaired autophagic flux and the accumulation of damaged mitochondria. Modulating LACTB expression to enhance PISD activity and PE production helps maintain mitochondrial homeostasis and the integrity of aging cardiomyocytes. These findings highlight the disruption of mitochondrial lipid metabolism as a central mechanism driving the accumulation of dysfunctional giant mitochondria in aged cardiomyocytes and suggest that inhibiting LACTB expression could serve as a potential therapeutic strategy for mitigating cardiac aging and preserving mitochondrial function.