Digital　holography　is　an　emergent　imaging　technique　which　provides　complete　three-dimensional　information　of　an　object.　An　off-axis　Fresnel　digital　holographic　system　with　pre-magnification　is　built.　Experimental　investigations　have　been　performed　for　a　USAF　test　target.　By　using　four　autofocusing　algorithms　the　optimal　recording　distance　of　the　recorded　hologram　is　determined　accurately.　The　experimental　results　show　that　the　variance　algorithm　is　the　most　effective　one　for　digital　holographic　microscopy.　Based　on　the　obtained　recording　distance,　the　phase　image　of　the　investigated　microscopic　object　is　reconstructed　accurately　by　associated　applying　the　automatic　phase　aberration　compensation　procedure　proposed　by　T.Colomb　with　the　manual　adjustment　method.　The　lateral　resolution　with　better　than　2.38　mu　m　and　the　axial　resolution　of　10nm　has　been　achieved　experimentally.