The deficiency of mixed discrete variables for the current optimal design is analyzed.A variable transformation method for mixed discrete variables is proposed,that is nominal variables are set,and software or algorithm of continuous variables could be called for mathematical model.The variables transformation program is written according to their relationship with real variables.Then,the nominal variables are put into the objective function and constraint condition,and the optimal solution is got.The variables of the optimal solution and real variables have the same type,so,it needn’t to be rounded and ensured the accuracy of the optimization.The variable transformation method is applied into optimization design of spur gear transmission,and compared with the existent ordinary optimization methods.The obtained results show that the variable transformation method is simple and feasible and the optimum solution of the variable transformation method is superior to solutions of other algorithm.