Considering the space power conditioning unit (PCU) needs high reliable design,especially for it works in harsh environment, a robust design and optimal method is proposed. Firstly,the mathematical model of sequential switching shunt regulator(S3R) is shown to be suitable for regulated bus power systems. Secondly, optimize the bus capacitor parameter by the parameter sweep analysis. Thirdly, when all the components change simultaneously in the worst case, the performance of the S3R circuit meets the design requirements and the shunt component meets the derating requirements in the Monte Carlo analysis and stress analysis. Finally, simulate two failure modes: the load changes in pulse mode and the current supply of the second-stage solar array is zero,the simulation results show that the output of the bus voltage is stable and ripples in the specified range.