To　achieve　energy　saving　and　emission　reduction　for　vehicle　diesel　engines,　the　organic　Rankine　cycle　(ORC)　was　employed　to　recover　waste　heat　from　vehicle　diesel　engines,　R245fa　was　used　as　ORC　working　fluid,　and　the　resulting　vehicle　diesel　engine-ORC　combined　system　was　presented.　The　variation　law　of　engine　exhaust　energy　rate　under　various　operating　conditions　was　obtained,　and　the　running　performances　of　the　screw　expander　were　introduced.　Based　on　thermodynamic　models　and　theoretical　calculations,　the　running　performance　of　the　vehicle　diesel　engine-ORC　combined　system　was　analyzed　under　various　engine　operating　condition　scenarios.　Four　evaluation　indexes　were　defined:　engine　thermal　efficiency　increasing　ratio　(ETEIR),　waste　heat　recovery　efficiency　(WHRE),　brake　specific　fuel　consumption　(BSFC)　of　the　combined　system,　and　improvement　ratio　of　BSFC　(IRBSFC).　Results　showed　that　when　the　diesel　engine　speed　is　2200　r/min　and　diesel　engine　torque　is　1200　N　m,　the　power　output　of　the　combined　system　reaches　its　maximum　of　approximately　308.6　kW,　which　is　28.6　kW　higher　than　that　of　the　diesel　engine.　ETEIR,　WHRE,　and　IRBSFC　all　reach　their　maxima　at　10.25%,　9.90%,　and　9.30%,　respectively.　Compared　with　that　of　the　diesel　engine,　the　BSFC　of　the　combined　system　is　obviously　improved　under　various　engine　operating　conditions.