Metallized　film　capacitor　(MFC)　is　one　of　the　key　components　in　power　electronic　converters,　accounting　for　a　large　proportion　of　failures.　However,　the　time-varying　external　stress　in　long-term　mission　profile　and　time-varying　internal　stress　due　to　the　degradation　of　MFC　are　not　well　described　by　the　conventional　reliability　evaluation　method,　which　leads　to　a　large　deviation　between　the　evaluation　results　and　the　engineering　practices.　Therefore,　an　improved　reliability　evaluation　method　considering　time-varying　stress　mission　profile　and　aging　process　for　MFC　is　proposed.　First,　this　article　analyzes　the　influence　of　harmonics　and　provides　a　more　accurate　lifetime　model　under　constant　stress　condition,　compared　with　the　conventional　method.　Second,　the　subsection　integral　accumulated　damage　model　is　proposed　to　transform　the　long-term　time-varying　stress　mission　profile　to　the　short-term　ideal　constant　stress　condition,　solving　the　reliability　evaluation　under　time-varying　external　stress　condition.　And　then,　an　improved　aging　model　is　proposed　to　describe　the　nonlinear　variation　of　equivalent　series　resistance　(ESR)　and　internal　temperature　rise　due　to　the　degradation　process　of　MFC.　Furthermore,　Monte　Carlo　simulation　is　applied　to　analyze　the　difference　of　individual　capacitor,　obtaining　lifetime　distribution.　Finally,　a　practical　application　example　of　modular　multilevel　converter　(MMC)　system　is　implemented　to　verify　the　superiority　of　the　improved　method.