A polymorph of (R) methyl-2-(3-aminopiperidin-1-yl)-3-(2-cyanobenzyl)-4-carbonyl-3,4-dihydrothioph ene[3,2-d]pyrimidine-6-carboxylic acid maleate and a preparation method therefor. The polymorph is a crystal having high stability and low hygroscopicity, and the crystal form is selected from crystal form A, crystal form B and crystal form C. Furthermore, the crystal form has a strong in-vivo hypoglycemic activity and can be used for the preparation of a novel drug for the treatment or prevention of diabetes mellitus type H and/or complications of diabetes mellitus type H.
