The　amount　of　low　temperature　heat　resources　is　very　huge,　efficient　utilization　that　energy　is　very　important　issue　for　improving　energy　efficiency,　saving　energy　and　protecting　environment.　Due　to　the　small　available　energy　of　low　temperature　heat　source,　how　to　improve　thermodynamic　efficiency　is　the　key　problem.　In　this　paper,　the　thermodynamic　model　of　low　temperature　thermal　power　conversion　system　based　on　organic　Rankine　cycle　was　described　firstly.　Turbine,　single　screw　and　piston　expanders　were　briefly　described.　R123,　R245fa　and　R134a　were　chose　as　working　fluid　because　of　quite　different　critical　temperature.　Based　on　this　model,　the　influence　of　thermodynamic　property　of　organic　working　fluid　on　the　efficiency　of　low　temperature　thermal　power　conversion　system　was　discussed.　The　calculating　result　showed　that　R123　is　the　best　choice　if　no　considering　the　impact　of　expander　types　and　that　R245fa　is　the　best　choice　if　considering　the　impact　of　expander.　This　conclusion　indicated　that　it　is　very　important　to　investigate　the　match　relationship　between　working　fluid　and　expander.　Moreover,　single　screw　expander　was　proved　to　be　more　suitable　than　turbine　and　piston　expanders　for　low　temperature　heat　power　conversion　system.