This study is a phase III study in subjects with T2DM. As designed, the study will start with a 4-week, single-blind, placebo run-in period based on diet and exercise interventions for screening eligible subjects. The eligible subjects are randomly assigned to HMS5552 75mg BID group or placebo BID group with ratio 2:1 to receive a 24-week double-blind treatment. Then all subjects receive 28-week open-label treatment of HMS5552 75mg BID . After 52-week treatment, all investigational drugs should be discontinued and followed by 1-week for safety evaluation.