In traditional Chinese medicine, Fuzi is widely used as an antitumor agent or an adjuvant medication combined with radiotherapy and chemotherapy, but its mechanism remains unclear. In this study, we investigated anti-tumor and immunoregulation efficacy of Fuzi combined with radiotherapy in mice with Lewis lung cancer (LLC). We found that Fuzi combined with radiotherapy significantly inhibited the growth of LLC, promoted the apoptosis of cancer cells, and prolonged the survival of mice with LLC. Mechanistically, we found that Fuzi decreased the proportion of Treg cells, reduced serum levels of cytokines such as interleukin (IL)-10 and transforming growth factor (TGF)-β, and downregulated the expression of programmed death ligand-1 in mice with LLC subjected to radiotherapy. This study suggests that Fuzi has immunomodulation function to act as radiosensitizer and improve radiotherapy against lung cancer.