A　booster-driven　loop　free　cooling　system　plays　a　crucial　role　in　enhancing　data　center　energy　efficiency　and　reducing　emissions　due　to　the　special　booster　with　low　pressure　ratio　as　a　vital　driving　component.　And　the　impact　of　lubricant　and　its　fraction　on　the　booster　operation　are　the　key　points　under　low　pressure　ratio,　as　well　as　lubricant　distribution.　In　this　paper,　the　refrigerant/lubricant　two-phase　mixing　flow　in　the　working　chamber　of　a　booster　was　investigated　under　low　pressure　ratio　for　data　center　free　cooling.　The　volume　of　fluid　(VOF)　multiphase　flow　model　combining　the　working　chamber　of　the　rotary　booster　and　discharge　valve　was　built　up　to　analyze　the　dynamic　characteristics　of　the　booster　under　free　cooling　conditions.　The　effect　of　lubricant　fraction　and　its　working　frequency　on　the　operating　indexes　(exhaust　temperature/pressure,　mass　flow　rate,　volumetric　efficiency,　indicated　specific　work,　energy　efficiency　ratio)　were　studied.　The　lubricant　distribution　and　its　changes　for　different　angles　and　frequencies　were　presented.　The　results　indicate　that　it　can　improve　the　thermal　performance　of　the　booster　effectively　with　appropriate　volume　fraction　of　lubricant.　The　lubricant　in　the　twophase　flow　reduces　the　inner　temperature　and　pressure　in　the　booster　working　chamber,　which　changes　the　exhaust　pressure　and　volumetric　efficiency　as　well　as　the　cooling　capacity.　The　exhaust　pressure　and　the　mass　flow　rate　increase　notably　for　the　refrigerant/oil　mixture　with　the　increasing　lubricant　volume　fraction,　while　other　performance　indexes　decrease.　The　exhaust　pressure　and　the　mass　flow　rate　increase　by　0.51　%　and　47.14　%,　separately,　when　the　lubricant　volume　fraction　raises　from　0　to　5　%　at　operating　frequency　of　50　Hz.　Whereas,　the　exhaust　temperature,　refrigerant　mass　flow　rate,　volumetric　efficiency,　indicated　specific　power,　cooling　capacity　and　EER　decrease　by　18.11　%,　4.81　%,　47.14　%,　38.94　%,　4.81　%　and　26.36　%,　respectively.　The　investigations　can　support　the　lubrication　and　sealing　of　the　booster　with　low　pressure　ratio　for　free　cooling　to　reach　efficient　and　reliable　operation.