Multiferroic Janus nanofibers consisting of barium titanate BaTiO3 and cobalt ferrite CoFe2O 4 have the magnetoelectric effect between piezoelectric and magnetostrictive phases. By applying an external magnetic field on Janus nanofibers, the dipole moment in piezoelectric phase would be formed and produce the electric field. Since poly(N-vinylcarbazole) (PVK) polymer has anisotropic property, the birefringence can be induced by the electric field. We generalize the magnetic field-birefringence quadratic equation and coupling constant for PVK polymer with Janus nanofibers. Birefringence microscopy is used to observe the PVK birefringence under an external magnetic field from 0 to 2500 Oe. The numerical results show that the birefringence variation of the fourth digit after the dot can be obviously detected by our microscope for the distance within 15 mum between two nanofibers in PVK polymer.