In engineering design there exist the complicated problems on continuous and discrete variables. The solution method (and the optimization theory) in classical methematics were tried to solve such problems. However, it was difficult to get the satisfied results. The processing method of discrete variables is a better approach to solve them. This paper discusses the process of optimum check and solution in the processing method for discrete variables. The examples have verified that this method has the advantages of simple program, quick calculation and high accuracy, and is of practical importance to the optimal processing mixture variables in engineering design.