In modular multilevel converter (MMC), the existence of second- and other even-order harmonics in the circulating currents can have a negative impact on the operation of the converter. To mitigate the dominant second-order harmonic circulating current, the harmonic extraction method plays an important role in steady-state accuracy and dynamic performance of circulating current suppression. This article proposes a time-delayed filter-based approach to suppress the circulating current. Operating principles and filtering characteristics of the time-delayed filter are investigated. The proposed solution achieves accurate suppressing and frequency-deviation adaptation with simple implementation. Experiments from a single-phase grid-connected MMC have been conducted to validate the correctness of the proposed solution.