Birefringent materials with high optical anisotropy have been identified as a research hotspot owing to their significant scientific and technological significance in modern optoelectronics for manipulating light polarization. Researchers studying borate systems have discovered that adding π-conjugated units placed in parallel can significantly increase the birefringence of crystalline solids; some examples include [BO3] units, [B2O5] units, and [B3O6] units. However, there are not many borates with strictly parallel configurations of π-conjugated [B2O5] units. In this study, a new bimetallic borate Sr2Cd4(B2O5)3 with near-parallel arrangement of π-conjugated [B2O5] units was discovered. Sr2Cd4(B2O5)3 possesses the maximum number density of [B2O5] units, shortest dihedral angle of [B2O5] units (between the two [BO3]), and largest degree of [CdO6] octahedral distortion among all the currently known Sr-Cd-B-O tetragonal system borates, making it demonstrate a large birefringence of 0.102 at 532 nm. Theoretical analysis proves that π-conjugated [B2O5] anions are the primary source of the large birefringence of Sr2Cd4(B2O5)3.