Ionizing radiation in space, radiation devices or nuclear disasters are major threats to human health and public security. Expanding countermeasures for dealing with accidental or occupational radiation exposure is crucial for the protection of radiation injuries. Circulating microRNAs (miRNAs) have emerged as promising radiation biomarkers in recent years. However, the origin, distribution and functions of radiosensitive circulating miRNAs remain unclear, which obstructs their clinical applications in the future. In this study, we found that mmu-miR-342-3p (miR-342) in mouse serum presents a stable and significant decrease after X-ray total-body irradiation (TBI). Focusing on this miRNA, we investigated the influences of circulating miR-342 on the radiation-induced injury. Through tail vein injection of Cy5-labeled synthetic miR-342, we found the exogenous miR-342-Cy5 was mainly enriched in metabolic and immune organs. Besides, the bioinformatic analysis predicted that miR-342 might involve in immune-related processes or pathways. Further, mice were tail vein injected with synthetic miR-342 mimetics (Ago-miR-342) after irradiation to upregulate the level of miR-342 in circulating blood. The results showed that the upregulation of circulating miR-342 alleviated the radiation-induced depletion of CD3⁺CD4⁺ T lymphocytes and influenced the levels of IL-2 and IL-6 in irradiated mice. Moreover, the injection of Ago-miR-342 improved the survival rates of mice with acute radiation injury. Our findings demonstrate that upregulation of circulating miR-342 alleviates the radiation-induced immune system injury, which provides us new insights into the functions of circulating miRNAs and the prospect as the targets for mitigation of radiation injuries.