The anisotropically absorbing bacteriorhodopsin (BR) molecules can be approximated by linear oscillators. BR film is isotropic in its initial state because of the random distribution of the BR molecules. Upon excitation by a linearly polarized light,those molecules with the dipole moment aligned closed to the polarization direction of the excitation light are dominantly pumped to the intermediate state and produce photoisomerization. However,those with the dipole vector in the perpendicular direction of the pumping beam have a little probability occurring the above change. Due to the change of molecular structure,the refractive index and absorption coefficient of the BR molecules change,then the macroscopic anisotropy was induced as a result. The photoinduced birefringence of the BR film is measured by the orthogonal polarization detection,the experimental results are basically consistent with the simulated results,which are obtained by utilizing the two-state model of BR molecule photocycle.