Objective To investigate the effects of collagen peptides on the immune function of mice under the condition of X-ray irradiation combined with simulated weightlessness. Methods Mice were randomly divided into control group, modelling group and collagen peptide group. Mice in collagen peptide group were intraperitoneally injected with collagen peptide (600 mg/kg) once a day from the first day of the experiment, while mice in the other two groups were intraperitoneally injected with normal saline. On the fourth day of the experiment, mice in the modelling group and collagen peptide group were simultaneously exposed to X-ray irradiation (2 Gy) and hindlimb-unloaded simulated weightlessness by tail-suspension. On the 10th day of the experiment, the mice were terminated by cervical dislocation. Automated hematology analyzer was used to detect Leukocyte classification of peripheral blood. Splenic lymphocyte subsets, cell cycle and apoptosis of bone marrow cells were analyzed by flow cytometry. The expressions of 19 plasma cytokines were tested with liquid suspension chips. Results Compared with the control group, the modelling group had a significant reduction in the total number of white blood cells and lymphocytes in the peripheral blood, the total number of splenocyte and the number of T cells, CD4+ and CD8+ T cells, B cells, and natural killer (NK) cells in the spleen, an decrease in 18 cytokines in the plasma, and an increase in myelocyte apoptosis in mice of the modelling group. Compared with the modelling group, most immunological parameters had improved in the mice of the collagen peptide group except some cytokines. Conclusion Collagen peptides can effectively improve the immune function of mice under the condition of X-ray irradiation combined with simulated weightlessness.