The idea of the degree of membership is the basic idea of fuzzy mathematics. The key to solve practical problems using fuzzy mathematical method lies in establishing the actual membership function. However, how to correctly determine the membership function is still not yet fully resolved so far. Therefore, a method for determining the membership function based on the separability measure is presented. This method can determine the division of fuzzy sets by using the separability on various features between any two categories, and then determine the membership function to describe each fuzzy set. After that, an experiment is carried out in the track circuit fault diagnosis, and the result shows the effectiveness of the proposed method.