Objective To compare the efficacy and metabolic effects of pioglitazone-metformin and basic insulin therapy on type 2 diabetes mellitus (T2DM) patients with overweight or obesity and poor blood glucose control. Methods A total of 153 T2DM patients with overweight or obesity and poor blood glucose control were enrolled in this study. They received treatment with pioglitazone-metformin (pioglitazone-metformin group, n=77) or insulin glargine (basal insulin group, n=76) for 6 months in addition to their previous oral hypoglycemic drugs. At baseline, 3 months and 6 months after treatment, glycosylated hemoglobin (HbA_1c), fasting blood glucose (FBG), 2 h post-prandial blood glucose (2hBG), fasting insulin (FINS), 2 h post-prandial insulin (2hINS), fasting C peptide (FCp), 2 h post-prandial C peptide (2hCp), body mass index (BMI), total cholesterol (TC), triglyceride (TG), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C), and hepatic fatty degeneration (expressed as controlled attenuation parameter[CAP] value) were observed and recorded. Results At baseline, there were no significant differences in gender, age, BMI, HbA_1c, FBG, 2hBG, FINS, 2hINS, FCp, 2hCp, TC, TG, HDL-C, LDL-C, CAP value, underlying diseases, or concomitant medicine between the two groups (all P>0.05). At 3 and 6 months after treatment, the HbA_1c, FBG and 2hBG levels were significantly decreased versus those at the baseline in the two groups (all P<0.01), but there were no significant differences between the two groups (all P>0.05). Compared with the basal insulin group, the FINS, BMI and CAP values were significantly decreased in the pioglitazone-metformin group 3 and 6 months after treatment (P<0.05, P<0.01). After 6 months of treatment, there were no significant changes of blood lipid levels in both groups. Conclusion In T2DM patients with overweight or obesity and poor blood glucose control, adding pioglitazone-metformin and basal insulin to their previous oral hypoglycemic drugs has similar hypoglycemic effect. However, patients receiving pioglitazone-metformin have better metabolic benefits such as lower BMI, lower insulin and improved hepatic fatty degeneration.