Based　on　the　point　spread　function　of　the　off-axis　Fresnel　digital　holographic　system　with　pre-magnification,　the　phase　aberration　introduced　by　microscope　objective　is　obtained.　Using　a　collimated　light　as　reconstructing　wave,　the　phase　aberration　introduced　by　the　difference　between　the　reconstructing　wave　and　the　reference　recording　wave　is　analyzed.　This　method　is　very　simple,　and　it　is　very　different　from　the　one　proposed　by　T.　Colomb　et　al.　The　phase　mask　that　can　be　used　to　compensate　automatically　the　phase　aberration　in　the　phase　reconstruction　is　obtained.　According　to　the　principle　of　digital　holography,　it　must　be　pointed　out　that　for　Fresnel　digital　holography,　the　recording　distance　must　be　determined　accurately　before　compensating　automatically　the　phase　aberration.　This　is　done　by　an　automatic　focusing　procedure,　which　is　based　on　image-gray-entropy-method.　The　simulation　result　for　a　special　three-dimensional　micro　object,　which　is　polluted　by　a　random　noise,　is　presented.　The　percentage　error　of　the　reconstructing　distance　obtained　by　the　focusing　procedure　is　below　0.6　for　SNR　=　25.　Then　an　automatic　aberration　compensation　procedure,　which　is　the　same　as　that　one　proposed　by　T.　Colomb　et　al.,　is　applied　to　reconstruct　the　phase　image.　The　results　show　that　for　a　weak　noise　the　above　method　is　very　effective;　for　a　stronger　noise　the　procedure　described　here　is　applied　iteratively,　starting　from　the　initial　values　provided　by　the　first　evaluation;　while　for　a　very　strong　noise　the　procedure　fails　at　all.　Moreover,　after　applying　a　median　filter　to　the　primary　reconstructed　phase　image,　the　aberration　of　the　phase　image　obtained　by　further　:iteration　decreases,　at　the　same　time　the　noise　is　strengthen.