The self-diffraction characteristics of bacteriorhodopsin(bR) film illuminated by weak light were studied. The model describing the distribution of the light intensity of the amplitude grating induced by interference light in bR film under weak light was given,and the distribution of the transmitted light intensity y from the amplitude grating was described approximately as ysin^ax,which was in agreement with the experiment. The relationship between y and α was obtained,which indicated that giveing the wavelength of incident light,the weaker the incident light intensity would become,the bigger a would get.When the incident light intensity was 0.072/T(mW/cm²),α equaled to 1.From different incident intensities and incident angles,the self-diffraction efficiencies were calculated,from which it could be found that the optimal incident angle was 2° corresponding to the maximum diffraction efficiency.