To compare the suppressive effect of electroacupuncture(EA) at different stages of tumor development on tumor growth in 4T1 tumor-bearing mice，and clarify the optimal time window for tumor suppression by EA at ST36，to investigate the immunological mechanism of EA in treating tumors.Methods:BALB/c mice were randomly divided into control，model，positive and EA groups.According to the time window of EA intervention，EA groups were divided into 4 groups(EA1，EA2，EA3 and EA4).The orthotopic breast cancer model was established by implantation of 4T1-luc cells in the second breast pair on the right side of mice.EA(0.1 mA，2/15 Hz) was applied to bilateral ST36 for 30 min，once every other day.Mice in the positive group were intraperitoneally injected with gemcitabine.The experiment was repeated and verified in control，model，EA3，EA4 and positive groups.Vernier calipers and live imaging system were used to determine the tumor size.The proportions of CD8+T cells and natural killer(NK) cells in the blood were detected by flow cytometry.Results:The tumor size in positive，EA4 and EA3 groups were decreased compared with the condition in model group at 16 d，19 d and 22 d separately(P<0.05 or P<0.01)，and the tumor-suppressive effect lasted to the end of observation.The tumor weight of EA4 and positive groups was lower than that in model group(P<0.05 or P<0.01).Repeated experiment indicated that EA4 group displayed a stronger suppressive effect on tumor growth than EA3 group but with no significant difference.Compared with the condition in control group，the proportion of CD8+T cell and NK cell in the blood was reduced in model group，while the proportion was increased in EA3 and EA4 groups，and the mice in EA4 group had a higher proportion of CD8+T cell and NK cell in the blood.Conclusion:EA intervention after tumor formation displayed the strongest tumor-suppressive effect，which might be related to its improved anti-tumor immunity.