Abstract: Metallized polypropylene film (MPPF) capacitor is one of the weakest components in modular multilevel converters (MMC). The accurate assessment of its reliability is crucial for the safe and stable operation of flexible DC transmission systems. The melting of the polypropylene film during the MPPF capacitor's degradation will increase the equivalent series resistance (ESR), then increase the power loss of the capacitor and accelerating the aging process of it. To solve the problem, a method was proposed to characterize the increase of the ESR by utilizing the high frequency band energy of output voltage. The aging status of the capacitor can be judged by comparing the band energy after different service period. A failure detection model based on the band energy was finally established. The operation principle of capacitors in MMC system was studied, and the failure mechanism of the MPPF capacitor was analyzed. An MMC simulation model was established to find the relationship between the ESR and the high frequency band energy of capacitor's output voltage. A failure detection model of the MPPF capacitor was established, the feasibility and the validity of the proposed method and model were also verified through experiment. Compared with existing researches, this method can evaluate the failure status of the capacitor only with its output voltage. This method is also suitable for failure detection of capacitors in most switching circuits, and it especially solves the problem of high cost, complicated algorithm and poor universality in traditional methods. The intended outcome of this paper will lay scientific and technological foundation for condition monitoring and lifetime assessment of MPPF capacitors in modular multilevel converters.