A novel method is proposed to study optical imaging system using the information theory. The light intensity function is decomposed into linearity combination of a series of harmonic waves without negative value by improved Fourier transform. The normalized harmonic wave coefficient set of object is regarded as information source and that of image is regarded as information home. The optical information channel matrix is obtained by eigenvalue of imaging integral equations. Moreover, average mutual information and information channel capacity are calculated that can evaluate the information transmission ability of the optical system.