The　characteristic　of　gas　film　is　a　key　factor　in　the　performance　of　the　aerostatic　bearing.　Because　the　gas　film　flow　is　in　the　slip　regime,　influence　of　the　rarefied　effect　is　significant.　The　modified　Reynolds　equation　suitable　for　compressible　gas　in　the　rarefied　effect　is　deduced　through　introducing　the　flow　factor　in　the　rarefied　effect　to　the　Reynolds　equation.　Pressure　distribution,　capacity,　and　stiffness　of　the　gas　film　under　the　rarefied　effect　are　analyzed.　With　the　increase　of　gas　pressure,　the　gas　film　capacity　and　stiffness　of　bearing　would　also　increase.　However,　the　greater　the　gas　supply　pressure,　the　more　intense　the　gas　film　vibration,　so　it　was　important　to　select　a　reasonable　gas　supply　pressure　for　achieving　the　optimal　gas　film　characteristic.　Finally,　the　gas　rarefied　effect　is　verified　by　the　experiment　indirectly,　which　agreed　well　with　the　analytical　results　and　provided　a　theoretical　guidance　for　the　machining　accuracy　of　the　machine　tool.