Capacitors are widely used in power electronic converters to buffer the pulsation power, filter the harmonics and support voltage for stable operation. For these applications where single capacitor can not fulfill the voltage rating or capacitance requirements, capacitor bank is always used as the energy buffer by connecting several capacitors in parallel for larger capacitance, or in series for higher voltage rating. With more stringent constrains on volume for high power density applications, the thermal coupling effect becomes more severely which accelerates a portion of cells aging, therefore, reliability should be considered into the design phase. In this paper, considering the uneven temperature distribution in an ultra-compact capacitor bank, the lifetime of each cell is estimated at the beginning to reveal the reliability issue. Aiming to extend the lifetime of the capacitor bank with cost constraint, optimization solutions are proposed from the two aspects: rated lifetime allocation, and thermal loading distribution of each cell which are the significant factors to the lifetime. Simulation and experiment are conducted to verify the accuracy of the proposed solutions.