Objective To evaluate the immunotherapeutic effect of Bacille Calmette-Guérin (BCG) in type 1 diabetes mellitus (T1DM) mice and explore its mechanism. Methods Fifteen SPF grade male C57BL/6 mice were randomly divided into molding group (n=10, established T1DM model by intraperitoneal injection of streptozotocin 50 mg/kg)and control group (n=5, intraperitoneal injection of equivalent amount of citric acid buffer). Ten mice with random blood glucose≥16.7 mmol/L were divided into T1DM group and BCG group (5 each). Mice in BCG group were then subcutaneously injected with 1×10⁶ cfu/mouse of BCG at an interval of 4 weeks for 2 times, and the other two groups were injected with the same dose of phosphate buffer solution (PBS). During the experimental period (13 weeks), the body weight and food consumption of mice were monitored weekly, and blood glucose levels were measured by tail vein sampling. Oral glucose tolerance test (OGTT) was used to detect the glucose regulation ability of mice after glucose load. HE staining was used to observe the pathological changes of pancreatic tissue. Pancreatic insulin level was detected by immunohistochemistry (IHC). Serum C-peptide levels were detected by enzyme linked immunosorbent assay (ELISA). Real-time fluorescence quantitative PCR (qRT-PCR) was used to detect the transcription levels of cytokines in mice spleen. The proportion of regulatory T cells (Treg) in splenocytes was detected by flow cytometry. Results After twice immunization with BCG (13-week), the blood glucose level increased significantly of mice in T1DM group than that in control group (P<0.001); while in BCG group was still higher than that in control group, but was significantly lower than that in T1DM group (P<0.01). The oral glucose tolerance test (OGTT) showed that, the blood glucose levels of mice in the three groups increased rapidly after glucose load, and began to decline at 120 min, and decreased to normal level at 180 min in control group; Compared with control group, the blood glucose levels of mice in T1DM group and BCG group also decreased after 120 min, but were still higher significantly than in control group at 180 min (P<0.001), while the blood glucose level in BCG group was lower than that in T1DM group, but the difference was not statistically significant (P>0.05). At initial immunization (5th-week), compared with control group, the percentage of weight gain decreased in T1DM group and BCG group (P<0.0001), and the food consumption increased significantly (P<0.0001); After twice immunization with BCG (13th-week), compared with control group, the percentage of weight gain in T1DM group was still lower significantly (P<0.0001), and the food consumption was significantly higher (P<0.001).Compared with T1DM group, the percentage of weight gain in BCG group increased (P<0.001), and the food consumption was lower (P<0.001), and there was no significant difference compared with control group (P>0.05). The results of HE staining showed that, compared with control group, the islets of mice in T1DM group had obvious atrophy, while the morphology of islets of mice in BCG group was similar to that in control group, and the number of cells in islets was significantly higher than that in T1DM group.The insulin immunohistochemistry (IHC) staining showed that, compared with control group, the insulin-positive area in pancreas of mice decreased significantly in T1DM group (P<0.01), and although the insulin positive area in BCG group was lower than that in control group (P<0.05), but was still significantly higher than that in T1DM group (P<0.05). ELISA test showed that the contents of serum C-peptide in T1DM group and BCG group decreased obviously than that in control group (P<0.001, P<0.05), however, the content of serum C-peptide in BCG group was significantly higher than that in T1DM group (P<0.05). qRT-PCR showed that, compared with control group, the level of cytokine mRNA in T1DM group did not change (P>0.05); Compared with T1DM group and control group, the mRNA levels of IL-2, IL-10 and transforming growth factor-β (TGF-β) were significantly increased in BCG group (P<0.0001), but there was no change in mRNA level of γ-interferon (IFN-γ) (P>0.05). The results of flow cytometry showed that compared with control group, the percentage of Treg cells in T1DM group decreased (P<0.05), while compared with T1DM group, the percentage of Treg cells in BCG group increased obviously (P<0.05), and there was no statistical difference compared with control group (P>0.05). Conclusion BCG immunotherapy could induce the increase of Treg cells, regulate autoimmune response, and promotes insulin secretion by reducing pancreatic pathological damage and restoring islet cell function, thus reducing the blood glucose level of T1DM mice.