Abstract: In order to study the influence of electrical aging on the frequency domain spectroscopy (FDS) characteristics of the main insulation in large generator stator bars, the FDS tests of Vacuum Pressure Impregnation(VPI) less-resin insulation and Resin-Rich Molding (RRM) insulation, which were under different electrical aging states, were carried out, the microstructure and chemical composition of the main insulation specimen were analyzed by Scanning Electron Microscopy(SEM) and Fourier Transform Infrared Spectroscopy(FTIR) before and after electrical aging, and the complex permittivity was fitted and analyzed by the Havriliak-Negami relaxation model in order to research how the FDS changed with the electrical aging times. The results show that the insulation microstructure is destroyed and the quantity of polar groups increases after electrical aging, and the interface polarization and dipole turning-direction polarization are also enhanced, so that the dielectric properties of the VPI and RRM insulation rise significantly in the low frequency region after electrical aging, such as the real part ε′ and the imaginary part ε″ of the complex permittivity and the dielectric loss factor tanδ. VPI and RRM insulation systems are affected to different extent by electrical aging, and the destruction of the VPI insulation structure is more serious, while the polar group of RRM insulation is more than that of VPI. The analysis results of complex permittivity before and after electrical aging reveal that the interfacial polarization relaxation intensity Δε₂ is much larger than the dipole polarization relaxation intensity Δε₁. The Δε₂ dominates the whole relaxation polarization strength in the insulation, and the electrical aging degree of the main insulation can be analyzed by Δε₂. The research findings provide a theory reference for the aging state evaluation and the ageing mechanism analysis of the large generator stator bars main insulation.