Metallized　film　capacitor　(MFC)　selection　is　a　key　step　to　ensure　the　safe　and　reliable　operation　of　high-capacity　power　electronic　equipment.　However,　as　a　crucial　factor　leading　to　overstress　failure,　the　changing　of　stress　tolerance　boundary　caused　by　parameter　drift　and　insulation　deterioration　has　not　been　fully　considered　in　capacitor　design.　Therefore,　the　establishment　method　of　MFC　lifecycle　safe　operation　area　(SOA)　is　proposed　innovatively　in　this　article,　which　characterize　the　shrinking　of　safe　operating　boundaries　caused　by　electrothermal　coupling　and　capacitor　aging　during　long-term　operation.　First,　the　failure　mechanisms　and　electrothermal　coupling　of　MFC　are　discussed　and　the　modeling　process　of　initial　SOA　is　proposed　based　on　the　datasheet　and　dc　withstand　voltage　experiments.　In　addition,　considering　the　increase　of　the　hot-spot　temperature　and　the　decrease　of　breakdown　strength　due　to　equivalent　series　resistance　parameter　drift　and　dielectric　insulation　degradation　in　the　whole　lifecycle,　the　shrinking　of　MFC　operation　boundary　is　characterized　and　the　modeling　process　of　lifecycle　SOA　is　proposed　based　on　aging　model　and　lifetime　tests.　Finally,　taking　an　MMC　system　as　example,　the　MFC　selection　based　on　practical　operating　condition　and　lifecycle　SOA　is　performed　to　verify　the　validity　of　the　method.