Background: Lycium barbarum is a traditional Chinese medicine. Its pharmacological effects mainly rely on a component called Lycium barbarum polysaccharide (LBP). The present study aims to explore the mechanism by which LBP reduces radiation damage in X-ray-irradiated cultured mouse bone marrow mononuclear cells （BMNCs） and the potential involvement of apoptosis. Materials and Methods: Mouse BMNCs were cultured in vitro and exposed to radiation. After 24 hours of LBP treatment, BMNCs viability was detected by cck-8 method, apoptosis rate was examined by Flow cytometry (FCM), Mitochondrial membrane potential (MMP) fluorescence was detected by JC-1, and the expression of mitochondrial pathway-associated protein was measured by immunoblotting. Results: LBP significantly increased BMNCs viability and reduced the apoptosis rate of radiation-exposed BMNCs 24 h after treatment compared to the non-treated control group. In a mitochondrial membrane potential fluorescence assay, LBP reduced the radiation-induced decrease in mitochondrial membrane potential. Western blot analysis further proved this point, where LBP inhibited the release of cytochrome C from mitochondria and also inhibited the expression of caspase 9 and other mitochondrial-related proteins. Conclusions: The mechanism of LBP action in radiation-exposed mouse BMNCs cells seems to involve inhibition of the mitochondrial apoptosis pathway.