Objective To investigate the efficacy of metformin combined with insulin injection in the treatment of adolescents with type 1 diabetes mellitus (T1DM) based on the continuous scanning glucose monitoring system. Methods A total of 99 adolescents with T1DM were followed from January 2018 to May 2023. The observation group, comprising 48 adolescents, received a combination of insulin and metformin, while the control group of 51 patients was treated with insulin alone. The clinical data, monitoring indices and safety profiles of both groups, were compared. Results At the 3-month follow-up, the mean time in the target glucose range (TIR) was significantly higher in the observation group at 76.6%±9.1% compared to 65.9%±15.0% in the control group. The median time above target glucose range (TAR) in the two groups was 7.0% (3.0%-14.3%) and 21.0 % (12.0%-29.0%), respectively. The mean time below target glucose range (TBR) was 14.50%±7.13% in the observation group and 10.2%±6.8% in the control group. At the 6-month follow-up, the mean TIR in the observation group was 76.0 %±8.9 % compared to 65.5 %±14.3 % in the control group. The median TAR in the observation group was 8.0 %(3.3 %-13.8 %), while in the control group it was 24.0 %(15.0 %-29.0 %).The median TBR in the observation group was 14.4 %±5.9 % in contrast to 10.0 %±6.8 % in the control group. The differences in these variables between the two groups were statistically significant (all P<0.05). At the 3-month and 6-month follow-up, the median insulin dosage required by the observation group was lower than that of the control group (all P<0.05). At two follow-up visits, the median insulin dosage, median BMI, average TIR and average TBR of the patients in the observation group were all higher than their baseline level, and the median TAR level was lower than their baseline level, with statistical significance (all P<0.05). Gastrointestinal symptoms were reported in 4 cases (8.33 %) of the observation group, with no severe hypoglycemia observed. Conclusions The combination of metformin with insulin therapy significantly improved TIR in adolescents with T1DM. Although TBR level increases, this treatment regimen does not increase the risk of severe hypoglycemia. Metformin demonstrated a favorable safety profile, suggesting that the insulin-metformin combination is a viable treatment option for adolescents with T1DM.