Based　on　the　Fresnel　diffraction　theory,　the　point　spread　function　of　digital　holographic　system　with　pre-magnification　was　firstly　deduced　in　detail.　The　resolution　of　the　imaging　system　was　analyzed.　The　amplitude　distributions　of　the　point　spread　functions　of　the　microscopic　objective,　the　CCD　and　the　whole　system　were　simulated　respectively.　Then,　the　match　requirement　between　the　microscopic　objective,　and　the　CCD　was　discussed.　Some　ambiguities　in　existing　literatures　were　clarified.　The　results　show　that　making　the　imaging　resolution　of　MO　approached　the　limit　resolution　and　the　imaging　resolution　of　CCD　been　equal　to　or　higher　than　the　resolution　of　MO　slightly　by　chosing　the　recording　parameters　pertinently　are　optimizational　choices.　These　results　will　be　very　important　in　the　microscopic　digital　holography.