Owing to the tendency to form the glass state in growing crystals in the ternary tin(II) borate system, hitherto, very few single crystals in ternary tin(II) borate were reported. In this paper, a single crystal of Sn3B8O15 was obtained via a high temperature solution method in a closed system. The structure features a flexible fundamental building block [B8O18](12-) constructed by one [BO3](3-) unit and one [B7O16](11-) group that contains three hexatomic rings, which further connect to compose a two-dimensional (2)(infinity)[B8O15] layer structure. The connection mode and flexibility of the three hexatomic B-O rings were analyzed, verifying the structural diversity of the B-O configuration. Moreover, the synthesis, crystal structure, and various characterizations were comprehensively presented. Also, theoretical calculations were employed to discuss structure-property relationships, and we use the real-space atom-cutting techniques to explain the origin of the birefringence of Sn3B8O15.