A　ground　experiment　is　established　to　investigate　the　pressure　control　performance　of　thermodynamic　vent　system　(TVS)　with　HCFC123.　Different　influence　factors,　including　tank　pressure　control　bands,　circulation　volume　flow　rates,　and　heat　loads,　are　investigated　separately.　The　variations　of　tank　pressure　and　fluid　temperature　are　analyzed　in　different　operation　process.　To　compare　the　performance　of　TVS　with　that　of　direct　venting,　the　tank　heat　leakage　is　solved　with　a　quasi-steady　heat　transfer　model.　With　the　actual　penetration　heat　into　tank　determined,　the　performance　comparison　is　made　between　direct　venting　and　TVS.　The　results　show　that　the　increase　rate　of　tank　pressure　rises　with　the　heat　load　during　the　pressurization　process.　While　the　bulk　fluid　is　still　subcooled,　great　tank　pressure　control　and　fluid　cooling　could　be　obtained　by　increasing　the　circulation　flow　rate　in　the　mixing　injection　process.　With　the　cold　capacity　generated　by　the　throttling　process,　both　the　vapor　and　liquid　should　be　well　cooled.　For　the　case　of　No.3　to　4,　as　the　refrigeration　capacity　of　TVS　could　not　eliminate　the　accumulated　heat　load　timely,　the　fluid　has　a　temperature　increase　in　the　early　stage　of　throttling　process.　While　for　the　case　of　No.6,　with　the　fluid　being　cooled　sufficiently,　both　the　vapor　and　liquid　have　received　great　temperature　control.　Compared　with　the　direct　venting,　the　recovery　ratio　of　venting　gas　loss　generated　by　TVS　ranges　from　30%　to　145%,　which　shows　the　TVS　has　a　large　advantage　on　exhaust　saving.