for a parameter α that played an important role in the course of fuzzy decision tree creation ,there exists a specific interval, within which gradual increasing of the parameter α may have the same effect as the crisp decision tree post pruning on the size and test accuracy, and there exists a optimum value of α within this specific interval, When α gets the value, can make the fuzzy decision tree reach its performance optimal. To obtain α optimum value , this paper proposed a method of optimizing parameter based on genetic algorithm and proved the validity of the method through comparing with the relative experiment.