Abstract: In view of the problem that the existing research on reliability of modular multi-level converter（MMC） does not fully consider time-varying failure rate of redundant system, and lacks the evaluation method for renewal process, which can not scientifically measure the reliability of MMC, this paper proposes an MMC reliability modeling method considering the time-varying failure rate and N-step renewal process of redundant system. Firstly, from the perspective of redundancy degree and redundancy mode, the classification of redundant systems is refined. According to the classification above, the mathematical analytical relationship between reliability and failure rate is established, which indicates the characteristics of the time-varying failure rate of redundant system. It is pointed out that the failure rate of redundant system obtained by traditional methods is too small. Then, N-step failure rate is used to replace the time-varying failure rate of MMC redundant system. At the same time, the renewal process theory is introduced to deduce the mean time to failure（MTTF） and steady-state availability formula of MMC, so as to correct the problem that the calculation results of traditional methods are too high. Finally, the converter of half-bridge sub-module is taken as an example to verify the effectiveness and feasibility of the proposed method.