Abstract: Accurate and rapid calculation of the junction temperature fluctuations of modular multilevel converter（MMC）power devices accurately and quickly is the key to lifetime estimation and reliability studies. The impact of fundamental frequency junction temperature fluctuations on MMC power module lifetime estimation cannot be ignored under a long-time mission profile. Due to the DC bias characteristics of the MMC bridge arm current,the power module has thermal imbalance characteristics inside,and this paper proposes a lifetime estimation method of MMC power module considering the thermal imbalance characteristics. Firstly,the thermal imbalance characteristics of the MMC sub-module power devices are analyzed. A low-frequency thermal cycle extraction method for power devices considering electrothermal coupling is proposed,and a fast-iterative solution method for the fundamental frequency junction temperature fluctuation is proposed considering the device loss thermal imbalance characteristics. Then,the link between thermal cycling and device damage established by Bayerer’s model and Miner’s theory is used to realize the lifetime estimation of power modules of MMC systems. Finally,the proposed method is validated and analyzed using an example.