Abstract: Abstract: The mechanism of the internal structure and the insulation failure of RVV cable insulation are studied by the FFT infrared spectrometer. The test data shows that there are not alcohols, phenols, organic acids, alkyne and olefinic compounds and its characteristic peak is associated with functional groups or compounds. Due to the extension of standard thermal aging time, the components of cable insulation changes and the characteristic peak decreases gradually. The formation of the C=C conjugate structure and the O－H strong polar enhance the conductivity of the insulating layer during the process of thermal aging of RVV cable insulation. The deeper the thermal aging degree is, the easier the insulation failure of RVV cable insulation layer is.