Abstract: To apply frequency domain dielectric spectroscopy technology (FDS) to the non-destructive diagnosis of epoxy/mica insulation, a generator winding bar sample was given an accelerated multi-stress aging treatment by combined use of thermal, electrical, and mechanical (vibration) aging methods, and its FDS parameters at different aging periods were measured experimentally. Based on these PDF parameters, a double relaxation Cole-Cole model with modified DC conductivity was established and effects of the insulation aging on the complex dielectric constant and relaxation Cole-Cole model parameters were analyzed. The results show that the real part and imaginary part of the complex dielectric constant increases gradually with aging time, and the increase tendency is more significant in the low-frequency band at 0.001-1.000 Hz. The mechanism of relaxation is changed by the aging of insulation, leading to a new relaxation process. Because of the new relaxation, the description of relaxation in the double relaxation Cole-Cole model of the first two periods is incoherent with that of the last three periods. The test values of the dielectric spectrum can be well fitted by the double relaxation Cole-Cole model. With the aging, the DC conductivity and interfacial polarization increases quickly, and the interfacial polarization relaxation time constant tends to decrease. The study provides a new idea for aging status assessment of generator using FDS method.