Mixed-coordinated borophosphates containing π-conjugated [BO3] and non-π-conjugated [BO4] and [PO4] tetrahedra have been a research hot spot benefiting from their diverse structures and the presentation of pre-eminent optical performances. Through the high-temperature solution method, a new mixed-coordinated borophosphate K2PbB5P3O17 was synthesized. This compound exhibits an unprecedented [B5P3O21] fundamental building block consisting of apex-sharing [B3O7], [BO4], and [PO4] units, possessing a short ultraviolet cutoff edge of 234 nm as well as moderate birefringence (0.045@1064 nm). Compared with the tetra-coordinated borophosphate KPbBP2O8 with the same elemental composition, the birefringence of K2PbB5P3O17 achieves a great enhancement induced by the introduction of the planar [B2O5] unit.