Objective: In vitro and in vivo effects of ethyl acetate extract from Cremastra appendiculata (Cr Ap) on 4T1 breast cancer and its immune mechanism in mice bearing 4T1 breast cancer. Methods: MTT assay was used to measure the inhibition of proliferation of Cr Ap with different concentrations (1250ug/mL, 1250 ug/ml, 125ug/mL, 12.5ug/mL, 1.25ug/mL, 0.125ug/mL) on 4T1 breast cancer cells at 24h, 48h, 72h. 100 BABL/c female mice were implanted with tumor. After successful implantation, they were randomly divided into 5 groups: blank control group, model group, Adriamycin (ADM) positive control group, Cr Ap group and Cr Ap+ADM group, with 10 mice in each group. The mice were weighed every 7 days to draw the weight change curve; the tumor diameter and short diameter were measured every 3 days to draw the tumor curve; the tumor inhibition rate was calculated by peeling off the tumor of mice; some cancer tissues were stained with HE; the levels of Interleukin-2 (IL-2), Tumor necrosis factor-ɑ (TNF-ɑ), Interferon-γ (IFN-γ) and iInterleukin-10 (IL-10) were detected by Enzyme-linked immunosorbent assay (ELISA). Results: In MTT experiment, the inhibitory rate of Cr Ap with different concentrations ranged from 13.48% to 74.47%, of which Cr Ap (125μg/mL) had the strongest inhibitory effect at 72h, with the inhibitory rate of 74.74%. The weight of model group was significantly lower than that of blank group (P<0.01); at the same time, the weight of ADM group and Cr Ap+ADM group were significantly lower than that of model group (P<0.01), but the weight of Cr Ap group was significantly increased (P<0.01); in addition, the weight of Cr Ap group and Cr Ap+ADM group were significantly higher than that of ADM group (P<0.01). Compared with the model group, the tumor volume of all drug groups were significantly reduced (P<0.01). All drug groups have inhibitory effect on tumor of tumor-bearing mice. Compared with the model group, Cr Ap group and Cr Ap+ADM showed cytoplasmic rupture and nucleolysis, and cell density decreased, and there are necrotic areas of different degrees in HE staining of cancer tissue.And each administration group is increasing the number of immune cells in breast cancer. In cancer tissues, compared with the model group, the content of IL-2 and IFN-γin each drug group was significantly increased (P<0.01), and the content of TNF-α and IL-10was significantly decreased (P<0.01). Conclusion: In vitro, Cr Ap can inhibit the proliferation of 4T1 breast cancer cells. In vivo, Cr Ap can inhibit the growth of breast cancer, improve the general behavior of tumor-bearing mice, and increase the weight of tumor-bearing mice. The mechanism of Cr Ap anti-4T1 breast cancer may increase the expression of IL-2, IFN-γ and reducing TNF-α, IL-10 in cancer tissue. Furthermore, In addition, Cr Ap can enhance ADM's anti-4T1 breast cancer and immunoregulation effects in Cr Ap and ADM combined medication.