The excellent birefringent materials are needed for optical systems. Herein, we reported a new compound, the first tin borate chloride, Sn2B5O9Cl (SBOC) with a large birefringence (0.168 at 546nm) measured by the polarizing microscope. Its birefringence is 16times that of the isostructural Ba2B5O9Cl (BBOC) compound (0.010@ at 546nm). The results show that the birefringence enhancement originates mainly from the Sn2+ polyhedra. We propose that the birefringence can be enlarged by substituting the alkaline-earth metal cation by the Sn2+ cation in the isostructural borate with small birefringence. This strategy will guide the discovery of large birefringent materials in the future.