In　data　center,　using　outdoor　low-temperature　air　as　free　cooling　is　one　of　the　most　effective　ways　to　increase　the　energy　efficiency　of　the　heat　source　system.　At　the　same　time,　optimizing　the　indoor　air　organization　to　reduce　the　energy　consumption　of　fans　is　considered　as　another　effective　way　of　saving　energy.　In　this　study,　a　new　cooling　system　for　data　center　by　using　evaporation　and　condensation　of　CO2　as　free　cooling　is　proposed.　Also,　the　system　is　optimized　by　organizing　the　cooling　airflow　with　natural　convection　and　small　distributed　fans　on　the　indoor　side.　This　paper　mainly　focuses　on　optimization　of　cooling　airflow,　especially　that　around　racks.　CFD　is　used　to　analyze　the　temperature　distributions　and　flow　distributions　by　adjusting　power　of　small　distributed　fans　and　shape　of　the　cooling　unit.　The　results　show　that　small　distributed　fans　and　natural　convection　can　generate　the　desired　distribution　of　cooling　airflow.　©　2020,　Springer　Nature　Singapore　Pte　Ltd.