In order to improve the resolution of reconstruction image in digital holography, the traditional resolution enhancement methods are studied. Here, a resolution improvement method based on extrapolation add with interpolation is proposed. The recorded hologram is interpolated and extra-padded using image algorithm. An iterative process is applied by diffracting the wave-field between hologram plane and object plane. The interpolated and extrapolated hologram can be retrieved by this method. The interpolation process can increase the special sample frequency while the extrapolation can enlarge the numerical aperture. Simultaneously, limiting factors of interpolation and extrapolation are also discussed. We have compared the new method with interpolation or extrapolartion, respectively. The simulation and experimental results show that the resolution of reconstructed image is improved significantly by using this proposed method.