Diffraction imaging characteristics of a 90° geometry volume holographic imaging system are investigated. Volume holographic grating, used as a volume holographic imaging lens in the imaging system, is recorded via illuminating the recording medium with a spherical signal beam and a plane reference beam. According to Bragg selectivity of volume holographic grating, the Bragg mismatch of diffraction imaging in z direction significantly decreases diffraction efficiency. The point spread function (PSF) of volume holographic imaging system is employed to analyze diffraction imaging characteristics and estimate depth resolution. The results show that the observed diffraction field obviously changes with the displacement of point source in z direction.