This study evaluates the efficacy and safety and population PK of HMS5552 as add on to Metformin in type 2 diabetic adult subjects, there will be 2 groups, in the first 24 weeks, one group will receive HMS5552 plus Metformin, while the other group will receive placebo plus Metformin; after 24 weeks, all subjects received HMS5552 plus Metformin for 28 weeks.