When faults occur outside of the generator, it is possible for the impedance criterion based protection to have unwanted operations. In the light of the features revealed themselves during loss in field and the fault out of generator, a new fuzzy set theory based method to identify the kind of fault is given. By analyzing the characteristic quantity and choosing relevant membership function, corresponding fuzzy mathematical model is given. The effectiveness and reliability of the presented method are verified by the results of simulation with Matlab.