Aviation gear reducer usually has high requirements for weight, strength, reliablity and efficiency. Considering the random characteristic of working conditions, the difference of technological level and the fluctuation of mechanical parameters such as material strength, the traditional deterministic parameter optimization design can not meet the needs of engineering practice. On the foundation of traditional deterministic optimization model, the multi-objective robust design optimization model of gear drive system is developed in this paper based on the basic theory of robust design of 6 sigma, and the Pareto solutions are obtained using NSGA-II multi-objective optimization algorithm. Subsequently, robust optimization for a single-stage helical gear system in an aviation gear reducer is carried out. The results show that the robust design model presented in this paper gives full consideration to the impact of uncertainty on optimization and has certain value for engineering application.