BR-D96N is a genetic mutant of bacteriorhodopsin (BR) with obviously photochromic characteristic benefiting from the prolonged lifetime of the M state. According to the Kramers-Kronig relation the absorption spectrum change of a sample will cause the refractive index change. In this paper, the autors firstly measured the absorption spectra of the BR-D96N film before and after light irradiation. Then the refractive index change spectrum of the sample is calculated according to the Kramers-Kronig transformation. In order to directly measure the photoinduced refractive index change of the bacteriorhodopsin film, Michelson interferometer is used to measure the refractive index change at different wavelength. The experimentally measured values were compared with the theoretically calculated curve.