One　way　to　increase　the　thermal　efficiency　of　vehicle　diesel　engines　is　to　recover　waste　heat　by　using　an　organic　Rankine　cycle　(ORC)　system.　Tests　were　conducted　to　study　the　running　performances　of　diesel　engines　in　the　whole　operating　range.　The　law　of　variation　of　the　exhaust　energy　rate　under　various　engine　operating　conditions　was　also　analyzed.　A　diesel　engine-ORC　combined　system　was　designed,　and　relevant　evaluation　indexes　proposed.　The　variation　of　the　running　performances　of　the　combined　system　under　various　engine　operating　conditions　was　investigated.　R245fa　and　R152a　were　selected　as　the　components　of　the　mixed　working　fluid.　Thereafter,　six　kinds　of　mixed　working　fluids　with　different　compositions　were　presented.　The　effects　of　mixed　working　fluids　with　different　compositions　on　the　running　performances　of　the　combined　system　were　revealed.　Results　show　that　the　running　performances　of　the　combined　system　can　be　improved　effectively　when　mass　fraction　R152a　in　the　mixed　working　fluid　is　high　and　the　engine　operates　with　high　power.　For　the　mixed　working　fluid　M1　(R245fa/R152a,　0.1/0.9,　by　mass　fraction),　the　net　power　output　of　the　combined　system　reaches　the　maximum　of　34.61　kW.　Output　energy　density　of　working　fluid　(OEDWF),　waste　heat　recovery　efficiency　(WHRE),　and　engine　thermal　efficiency　increasing　ratio　(ETEIR)　all　reach　their　maximum　values　at　42.7　kJ/kg,　10.90%,　and　11.29%,　respectively.