The standard genetic algorithm is improved by introducing the engineering treatment method of design vector in order to solve the optimization problem with mixed-discrete variables. A program of improved genetic algorithm has been designed. It can be used to solve the optimal design problems with continuous variables, discrete variables or mixed-discrete variables. For a dimension chain, the fuzzy-robust design of dimension tolerance is discussed and a model of fuzzy-robust design optimization is established. The solution of established model is achieved by using the improved genetic algorithm and the robustness of the dimension tolerance has been improved. The example shows that the proposed method is effective in engineering design.