The modular DC solid-state transformer (DC-SST) undertakes the high-frequency isolation function is also called the DC solid-state transformer (DC-SST) [1]. However, most reliability evaluations are based on AC converters [2]-[4], and researches about the reliability evaluation of DC-SST are relatively few. In [5], an optimized design method for DC-SST based on SiC MOSFET has been given. Besides, the lifetime of Si IGBT and SiC MOSFET have been compared under the daily load curve of the distribution network. However, the lifetime assessment for DC-SST only considers the thermal stress of the power semiconductor devices (PSDs), and does not include PSD aging and capacitor reliability. In addition, limited to the simulation speed of the simulation software, [5] did not perform the reliability assessment of DC-SST under the annual mission profile. Therefore, the system-level reliability assessment of DC-SST considering aging information of PSDs and capacitors under the long-term mission profile needs to be resolved urgently. Since the DC-SST is the interface connecting the medium and low voltage distribution network, there is a large voltage level difference between the primary and secondary sides. To meet the voltage and current requirements of PSDs, the DC-SST usually adopts a modular structure. Redundancy is one important measure to improve the reliability of the modular converter, including the DC-SST. The results of the reliability evaluation are usually used to guide the redundancy design of the system. For instance, in [6], the optimal cell number for the cascaded H-bridge (CHB) and the cascaded neutral-point clamped circuit (CNB) has been determined. Based on the optimal cascaded cell number, mean time to failures (MTTFs) under different redundant cell numbers have been analyzed. In [7], the reliability of IGBTs and capacitors in modular multilevel converters (MMC) have been evaluated separately, and the evaluation results are used to guide the redundancy design of MMC. In [8], the reliability of MMC has been evaluated considering the relevance of sub-modules, and the optimal module redundancy configuration scheme of MMC has been proposed based on the evaluation results. It is worth noting that when evaluating reliability, the above-mentioned research is only based on the stationary operation of the converter, and it is assumed that power devices have a constant failure rate. In fact, the long-term mission profile (ambient temperature, load fluctuation, wind speed, etc.) changes drastically, which will seriously affect the lifetime of power devices and converters. Moreover, the aging of critical components such as PSDs and capacitors needs to be considered.