In　order　to　make　better　use　of　waste　heat　energy　from　engine　exhaust　gas,　the　waste　heat　recovery　system　based　on　organic　Rankine　cycle　(ORC)　was　presented,　a　single　screw　expander　was　used　as　the　power　output　device,　R245fa　was　used　as　working　fluid,　the　concept　and　calculating　method　of　waste　heat　recovery　utilization　rate　was　introduced.　On　the　basis　of　engine　test　results,　the　effects　of　working　fluid　evaporating　pressure　and　engine　speed　on　the　ORC　system　performance　were　analyzed　and　discussed,　and　then　the　optimal　value　of　working　fluid　evaporating　pressure　was　selected.　According　to　the　research　results,　when　the　working　fluid　evaporating　pressure　is　3.0　MPa,　ORC　waste　heat　recovery　system　can　work　properly　over　the　whole　range　of　engine　speed,　the　maximum　of　net　power　is　12.1　kW,　the　maximum　of　first　law　efficiency　is　11.27%,　the　maximum　of　second　law　efficiency　is　25.8%,　the　maximum　of　waste　heat　recovery　utilization　rate　is　8.9%,　waste　heat　energy　from　engine　exhaust　gas　can　be　recovered　effectively.