Robustness optimization to enhance the reliability of an engine mounting system (EMS) is necessary in the decoupling design of the EMS, because the stochastic factors or uncertainties will have a significant influence on the decoupling state of the system and decrease the reliability of the system. This paper carries out the robustness optimization on the decoupling design based on torque roll axis decoupling theory. After building the dynamic model of the EMS, the constraints and objective functions of the optimization are improved by the principle of Six Sigma. A complete process of robustness is built and the examples for three different types of EMS are calculated and analyzed. The results show that the quality levels have been enhanced by the robustness optimization, which means the design of EMS is more reliable than deterministic design. Although the degree of the decoupling state lowers slightly, by taking all the performance of the system into consideration, the decrease of decoupling is acceptable and the robustness design is valuable in the real engineering.