In　this　paper,　a　basic　ORC　system　is　proposed　to　recover　the　exhaust　gas　energy　of　vehicle　engines,　and　the　mathematical　model　of　the　ORC　system　is　built.　Thirteen　working　fluids　are　analyzed.　The　main　criterion　for　selecting　working　fluids　is　the　mass　flow　rate　and　the　heat　absorption　rate　under　the　same　power　output　of　single-screw　expander.　Then,　the　paper　presents　an　analysis　of　the　irreversibility　rate　and　thermal　efficiency　of　the　ORC　system　using　4　different　organic　working　fluids.