Abstract: Aiming at the problem of thermal aging of organic materials,a thermal aging detection method for organic materials based on the second harmonic nonlinearity effect was developed. Considering that the amplitudes of the fundamental harmonic and the second harmonic are both changed with the frequency,a nonlinear damage factor used for the thermal aging damage assessment of the organic materials was proposed. Simultaneously,the nonlinear damage factor decays with the increase of the thermal aging time that was discovered by nonlinear ultrasonic detection. Therefore,the nonlinear damage factor can be used in the thermal aging damage assessment of organic materials.