The first tin (II) borate bromide, Sn2B5O9Br (SBOB), as an outstanding bifunctional material with excellent linear and nonlinear optical (NLO) performance is synthesized successfully. As far as it is known, it exhibits the largest birefringence among the reported borates (0.439 at 546 nm), even over famous birefringent materials YVO4 and TiO2. Meanwhile, it also owns a strong second harmonic generation (SHG) response of 2.4 times that of KH2PO4. Theoretical calculations display that its incredible optical properties mainly originate from the extremely intensive stereochemical activity lone pair (SCALP) of [SnO7Br2] polyhedra, which results in the enhancement of birefringence and SHG response, increasing to 2.6- and 4.8-fold, respectively, from Sn2B5O9Cl (SBOC) to SBOB. These perceptions will guide and accelerate the exploration of excellent birefringence and NLO materials in tin (II)-based compound.