The present invention provides a pharmaceutical composition for the treatment of 2 type diabetes, wherein the pharmaceutical composition contains (R)-7-[3-amino-4-(2,4,5-trifluoro-phenyl)-butyryl]-3-trifluoromethyl-5,6,7,8-tetrahydro-imidazo[1,5-a]pyrazine-1-carboxylic acid methyl ester or its pharmaceutically acceptable salts and metformin or its pharmaceutically acceptable salts (such as hydrochlorate), preparation method thereof and method of treating 2 type diabetes with the composition.