Based on the point spread function of holographic system, the lateral resolution of digital holographic imaging system without any pre-magnification is studied. The expression of resolution limitation of holographic imaging system is thus presented. We investigate the possibilities to improve the lateral resolution. The simple experimental setup with an off-axis arrangement is built. By using a U.S. Air Force (USAF) test target as microscopic object, the recorded holograms are reconstructed digitally based on the principle of Fresnel diffraction. The lateral resolution of 2.76 \mum without any pre-magnification is demonstrated experimentally, which matches the theoretical prediction well.