The　MMC　topology　is　composed　of　large　quantum　series-connected　submodules　(SMs).　The　fault　of　any　SM　will　affect　the　normal　operation　of　MMC　and　may　even　lead　to　further　cascading　faults,　thus　reliability　is　one　of　its　important　issues.　The　empirical　threshold　in　the　existing　literatures　is　difficult　to　generalize.　To　cope　with　this　problem,　this　article　proposes　a　threshold　adaptive　diagnosis　strategy.　The　fault　is　detected　and　located　by　judging　the　generalized　capacitor　voltage　unbalance　degree　of　the　SMs　whether　exceed　the　threshold.　By　analyzing　the　diagnostic　threshold　of　MMC　under　different　operating　powers,　the　proposed　strategy　can　accurately　and　quickly　diagnose　SMs　open-circuit　fault　at　different　power　points.　Experimental　results　in　a　hardware-in-the-loop　(HIL)　platform　verify　the　effectiveness　of　the　proposed　strategy.　©　2022　IEEE.