Modular　multilevel　converter　(MMC)　has　a　high　probability　of　IGBT　open-circuit　fault,　while　reliability　is　one　of　the　main　challenges　facing　MMC.　The　sub-module　fault　is　one　type　of　common　faults　of　MMC;　the　timely　diagnosis　and　location　is　a　great　concern　for　the　operational　stability　of　MMC-HVDC.　In　this　paper,　in　view　of　the　single-tube　open-circuit　fault　of　MMC,　a　method　for　fault　diagnosis　and　location　is　proposed.　The　method　is　based　on　sampling　the　output　voltage　of　single　arm　which　reduced　computation　complexity.　With　the　purpose　of　fault　diagnosis　and　location,　the　sub-module　fault　characteristics　were　analyzed,　then　according　to　the　diagnosis　rules　to　achieve　the　fault　status　identification　of　a　bridge　arm.　A　simple　sequential　process　was　applied　to　realize　fault　location.　Based　on　PSCAD/EMTDC,　a　high-voltage　DC　simulation　system　with　an　11-level　modular　multilevel　converter　was　built.　The　final　simulation　confirms　that　the　proposed　schemes　can　achieve　the　timely　sub-module　fault　diagnosis　and　location　so　that　the　stable　operation　of　MMC-HVDC　can　be　guaranteed.　©　Springer　Nature　Singapore　Pte　Ltd.　2020.