This study was designed to investigate the effect and mechanism of astragaloside IV (ASIV) on mitochondrial morphology and function of rat cardiomyocytes under hypoxia/reoxygenation injury. H9c2 cells were divided into control group, hypoxia/reoxygenation (H/R) group, and H/R + ASIV group. Cell viability and lactate dehydrogenase (LDH) leakage were measured by cell counting kit-8 (CCK-8) and LDH assay kit, respectively. Oxidative stress levels, such as superoxide dismutase (SOD), glutathione (GSH), and malondialdehyde (MDA), were analyzed by commercial kits. Intracellular and mitochondrial reactive oxygen species (ROS) levels were detected by dihydroethidium (DHE) and MitoSOX. Changes of the mitochondrial membrane potential were detected using the fluorescent probe JC-1. Opening of mitochondrial permeability transition pore was examined via calcein acetoxymethyl ester (calcein-AM). Apoptosis was assessed using terminal-deoxynucleoitidyl transferase mediated nick end labeling (TUNEL) assay kit. To detect protein expression of dynamin-related protein 1 (Drp1), mitofusin1 (Mfn1), Mfn2, Bax, B-cell lymphoma-2 (Bcl-2), and cleaved cysteine-aspartic protease (caspase)-3, Western blot analysis was carried out. Compared with the control group, ASIV (100 μmol·L^-1) significantly improved H/R induced cell injury, LDH leakage, decrease of SOD activity, and GSH content, increase of MDA content and ROS content, loss of mitochondrial membrane potential, mitochondrial permeability transition pore opening, ROS production activation, mitochondrial fission/fusion imbalance, and cell apoptosis. In addition, the effect of ASIV against H/R injury was also verified on primary rat cardiomyocytes. The animal welfare and experimental process follow the rules of Animal Ethics Committee of Zhejiang Chinese Medical University. In conclusion, ASIV may play a protective role in mitochondria by regulating morphological dynamic stability and mitochondrial function, inhibiting excessive synthesis of ROS, improving the internal environment of oxidative stress, reducing cell apoptosis, and thereby protecting against cardiomyocytes' hypoxia/reoxygenation injury.