The modular DC solid-state transformer(DC-SST) is promising but restricted by limited reliability, which can be solved by redundancy. However, current redundancy designs are only based on the reliability evaluation in stable operation condition and seldom efficiency. In this paper, a redundancy design method of DC-SST is proposed considering both reliability and efficiency. The long-term mission profile and aging of power semiconductor devices(PSDs) and capacitors are considered in the reliability evaluation. Based on the result that the capacitor reliability dominates the system reliability, a simplified system reliability model is proposed. Besides, IGBT power loss considering the long-term mission profile is obtained, which dominates the system power loss. Finally, a multi-objective optimization is solved to obtain the optimal redundant number. Feasibility of the proposed method is verified by a prototype.