In　flexible　high　voltage　direct　current　(HVDC)　system,　aging　and　failure　of　the　submodule　components　in　modular　multilevel　converter　(MMC)　threatens　the　safe　and　stable　operation　of　the　system.　In　this　paper,　a　method　on　monitoring　parameters　of　capacitors,　IGBTs　and　diodes　in　submodule　(SM)　simultaneously　is　proposed,　which　presents　the　state　of　SMs.　According　to　the　relationship　between　the　state　of　switching　devices　and　the　voltage　of　capacitor　in　SMs,　a　mathematical　model　of　bridge　arm　voltage　in　MMC　is　established.　Then,　Kalman　filter　is　introduced　to　monitor　the　parameters　of　elements,　which　can　prevent　the　influence　of　sensor　measurement　noise　and　save　storage　space.　Next,　the　state　of　the　submodule　capacitor,　IGBTs　and　the　diodes　is　monitored　through　the　established　mathematical　model　and　repeating　the　Kalman　filter　algorithm　multiple　times.　Finally,　the　scheme　is　verified　by　building　an　MMC-HVDC　transmission　system　in　Power　Systems　Computer　Aided　Design　(PSCAD)　simulation　platform.　The　result　shows　that　the　estimation　error　is　less　than　1%,　which　is　satisfied　to　the　on-line　monitoring　in　practical　engineering　and　can　improve　the　reliability　of　the　system.　©　2020　IEEE.