An experimental system is built based on the nonlinear eddy current (NEC) detection method. Quantitative nondestructive evaluation of the plastic damage degree of structural materials, such as Q195 carbon steel and 304 stainless steel, is studied. It is found that structural materials′plastic damage degree has a certain linear correlation with the fundamental frequency′s amplitude and the third harmonic component′s amplitude in the nonlinear eddy current detection signal spectrum. The linear correlation is different from each other. The detection signal of Q195 carbon steel decreases with increasing damage degree, while the detection signal of 304 stainless steel does the opposite. By developing this experimental system, importing plastic deformation, nonlinear eddy current testing experiment and analysis of detection signal, it is found that the amplitudes of the fundamental and third harmonic components of detection signals show good correlation with the degree of plastic damage. The feasibility and efficiency of the proposed method for quantitative nondestructive evaluation of structural materials′plastic damage are also investigated.