Radiation-induced heart injury (RIHI) limits the dose delivery of radiotherapy for thoracic cancer. Shenmai injection (SMI) is reported to have potential cytoprotective properties and is commonly used in cardiovascular diseases. So, we aimed to investigate the potential protective effects of SMI treatment on RIHI. In this study, we established the RIHI model using Sprague-Dawley rats and H9c2 cell line. In vivo, the biochemical assay was used to measure serum cardiac injury-related biomarkers and echocardiography to evaluate heart function. The pathological analysis was also applied to observe the myocardial structural changes. In vitro, we further measured the cell viability and reactive oxygen species (ROS) levels after irradiation with or without SMI treatment. Our data showed the administration of SMI reduced the level of serum cardiac injury biomarkers and ameliorated cardiac dysfunction after irradiation in rats. Pathological analysis revealed that SMI mitigated cardiac structural damage, fibrosis, and macrophage infiltration. Besides, treatment with SMI increased cell viability and decreased excess ROS production after irradiation in vitro. Taken together, our study demonstrated the protective role of SMI treatment on RIHI by inhibiting oxidative stress and decreasing structural remodeling.