The　main　purpose　of　this　research　is　to　analyze　the　performance　of　an　evaporator　for　the　organic　Rankine　cycle　(ORC)　system　and　discuss　the　influence　of　the　evaporator　on　the　operating　characteristics　of　diesel　engine.　A　simulation　model　of　fin-and-tube　evaporator　of　the　ORC　system　is　established　by　using　Fluent　software.　Then,　the　flow　and　heat　transfer　characteristics　of　the　exhaust　at　the　evaporator　shell　side　are　obtained,　and　then　the　performance　of　the　fin-and-tube　evaporator　of　the　ORC　system　is　analyzed　based　on　the　field　synergy　principle.　The　field　synergy　angle　(beta)　is　the　intersection　angle　between　the　velocity　vector　and　the　temperature　gradient.　When　the　absolute　values　of　velocity　and　temperature　gradient　are　constant　and　beta　＜　90　degrees,　heat　transfer　enhancement　can　be　achieved　with　the　decrease　of　the　beta.　When　the　absolute　values　of　velocity　and　temperature　gradient　are　constant　and　beta　＞90　degrees,　heat　transfer　enhancement　can　be　achieved　with　the　increase　of　the　beta.　Subsequently,　the　influence　of　the　evaporator　of　the　ORC　system　on　diesel　engine　performance　is　studied.　A　simulation　model　of　the　diesel　engine　is　built　by　using　GT-Power　software　under　various　operating　conditions,　and　the　variation　tendency　of　engine　power,　torque,　and　brake　specific　fuel　consumption　(BSFC)　are　obtained.　The　variation　tendency　of　the　power　output　and　BSFC　of　diesel　engine-ORC　combined　system　are　obtained　when　the　evaporation　pressure　ranges　from　1.0　MPa　to　3.5　MPa.　Results　show　that　the　field　synergy　effect　for　the　areas　among　the　tube　bundles　of　the　evaporator　main　body　and　the　field　synergy　effect　for　the　areas　among　the　fins　on　the　windward　side　are　satisfactory.　However,　the　field　synergy　effect　in　the　areas　among　the　fins　on　the　leeward　side　is　weak.　As　a　result　of　the　pressure　drop　caused　by　the　evaporator　of　the　ORC　system,　the　diesel　engine　power　and　torque　decreases　slightly,　whereas　the　BSFC　increases　slightly　with　the　increase　of　exhaust　back　pressure.　With　the　increase　of　engine　speed,　power　loss,　torque　loss,　and　BSFC　increment　increase　gradually,　where　the　most　significant　change　is　less　than　1%.　Compared　with　the　diesel　engine　itself,　the　maximum　increase　of　power　output　of　the　diesel　engine-ORC　combined　system　is　6.5%　and　the　maximum　decrease　of　BSFC　is　6.1%.