Submodule(SM) capacitor is one of the critical components in modular multilevel converter(MMC) and will gradually age with the working time increasing, which brings potential challenges for the systems reliability. This article proposes a novel online submodule capacitance monitoring scheme. Under normal conditions, the estimated capacitor voltage stays in line with the measured capacitor voltage. However, when the capacitor is aged, there will be a deviation between the estimated and measured values. Based on the voltage variations on rising and falling edge of switching function, the capacitance of aging capacitor can be estimated in real-time. Meanwhile, the proposed capacitance monitoring scheme is based on the mathematical model of the systems and has good applicability. It has no any impact on the normal operation of the MMC systems, no special requirement for the modulation method and no need for additional hardware circuits.