Prediction of the birefringence values of borate is very essential for developing new optical materials in UV range. In this paper, the birefringence values of five lead borates, Pb8B9O21F, PbBiBO4, Pb3BO4F, Pb6B3O10Cl, and Pb2BO3F with network B-O structure or isolated BO3 groups, are calculated by the first principle method. The calculations show that PbBiBO4, Pb3BO4F, and Pb2BO3F have the large birefringence, greater than 0.1. Pb2BO3F, especially, is the first compound with large birefringence above 0.08 among positive uniaxial borate crystals. It is found that the parallel arrangement of fundamental building units is not the only light anisotropy active character. In the further research of Pb2BO3F, polarization disproportionation via a visualized model is first put forward for identifying the origin of large birefringence, which will be helpful to search for new optical materials with suitable birefringence.