Objective To study the clinical effects of sitagliptin phosphate combined with metformin in the treatment of patients with type 2 diabetes mellitus (T2DM) complicated mild obesity and its influence on islet β cell function. Methods 184 patients with T2DM complicated with mild obesity in the hospital from July 2018 to April 2022 were divided into group A (n=61, metformin), group B (n=61, sitagliptin) and group C (n=62, combined treatment) according to the random number table method. The three groups were treated for 3 months. The changes of blood glucose, blood lipids, adipokines and islet β cell function and the occurrence of adverse reactions were compared among the three groups after treatment. Results After treatment, the fasting blood glucose (FBG), 2h postprandial blood glucose (2hPG), glycosylated hemoglobin (HbA1c), triglyceride (TG), total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) were significantly decreased in the three groups (P<0.05), and the FBG, 2hPG, HbA1c, TG, TC and LDL-C in group C were lower than those in groups A and B (P<0.05). The fasting insulin (FIns) and islet β cell function index (HOMA-β) were significantly increased in the three groups (P<0.05) while the homeostasis model assessment of insulin resistance (HOMA-IR) was significantly decreased (P<0.05), and the fasting insulin in group A was lower than that in groups B and C, and the HOMA-β in group C was higher than that in groups A and B while the HOMA-IR was lower than that in groups A and B (P<0.05). The levels of leptin and resistin were significantly decreased in the three groups (P<0.05) while the adiponectin level was significantly increased (P<0.05), and the levels of leptin and resistin in group C were lower than those in groups A and B while the level of adiponectin was higher than that in groups A and B (P<0.05). There were no significant differences in body mass index (BMI) changes and adverse reactions during treatment among the three groups (P>0.05). Conclusion Sitagliptin phosphate combined with metformin for patients with mild obesity and T2DM can effectively reduce blood glucose, blood lipids and adipokines levels, and improve islet β cell function