The　organic　Rankine　cycle　(ORC)　can　be　used　to　recover　the　waste　heat　from　a　stationary　compressed　natural　gas　(CNG)　engine.　However,　the　exhaust　energy　rate　varies　with　engine　load,　which　can　influence　the　operating　performance　of　the　ORC　system,　therefore,　it　is　necessary　to　study　the　running　state　of　the　ORC　system.　In　this　paper,　first,　the　numerical　simulation　model　of　the　ORC　system　is　built　by　using　GT-Suite　software,　with　R245fa　selected　as　the　working　fluid　of　the　ORC　system.　The　boundary　conditions　of　the　numerical　simulation　model　are　specified　according　to　the　measured　data　obtained　by　the　stationary　CNG　engine　test.　Subsequently,　the　power　output　and　dynamic　characteristics　of　expander　are　analyzed　to　determine　the　running　state　of　the　ORC　system.　Investigations　indicate　that　the　fluctuation　of　refrigerant　mass　flow　rate　in　the　expander　is　obvious　in　the　engine＇s　low-load　regions　(from　20%　engine　load　to　40%　engine　load).　The　performances　of　ORC　system　and　stationary　CNG　engine-ORC　combined　system　(combined　system)　are　finally　investigated,　respectively.　The　results　show　that　the　thermal　efficiency　of　the　combined　system　can　be　increased　by　a　maximum　5.0%　(at　the　engine　rated　condition),　while　the　brake　specific　fuel　consumption　(BSFC)　can　be　reduced　by　a　maximum　4.0%　(at　the　engine　rated　condition).