Due to the saturation absorption characteristic of bacteriorhodopsin film, in holography, when the intensity of the recording light is higher than the saturation intensity, the distribution of the transmission of the holographic gratings deviates from the cosine shape. As a result, the steady level of diffraction efficiency is very low. Using the mutual suppression property of bacteriorhodopsin films between the violet light and the red light, the saturation intensity of the film at 633nm can be increased by adding an auxiliary irradiation of 405nm violet light, which results in the holographic recording being changed from the nonlinear region to linear region. Consequently the distribution of the transmission of the holographic gratings retrieves to the cosine shape, and the diffraction efficiency is improved to a high value. Base on this principle, a three-beam holography system is set up. The diffraction efficiency and the quality of the diffraction image are shown to be greatly improved by the auxiliary violet light.