 Screening will be made to select eligible participants before intervention. The trial will include 1-week run-in period of stable doses of metformin 1500mg and 48-week treatment period. After the run-in period, patients were randomly assigned to one of two groups for antihyperglycaemic therapies for a total of 48-weeks: saxagliptin and glimepiride. The treatment of saxagliptin will be initiated and maintained at 5mg every morning until the completion of the study. Glimepiride will be initially treated with 1 mg every morning, and further titrated up at 4-week intervals until to reach the target fasting blood glucose FPG.As to Glimepiride groups, if the fasting blood glucose dose not achieved the target after the maximal doses, maintain the maximal doses6mg per day until the completion of the study.Meanwhile, participants in both groups should take metformin 1500mg per day throughout the whole 48 weeks. At the end of the study, data will be collected and analyzed. 