In this paper, the discriminant analysis method is introduced to identify the aging stages of oil-paper. An identification method based on Fisher discriminant function and multiple PD characteristic parameters is proposed. The 30 vectors distracted from 6 samples at 5 different aging stages are used as reference samples to obtain Fisher discriminant functions. The characteristic vector with eleven-dimension is mapped to two-dimension and the other 30 vectors are analyzed as samples need to be identified using discriminant functions. It is concluded that a reasonable result can be achieved using the Fisher discriminant functions.