In this study, the absolute value of the change rate of the real part of the complex permittivity at f=10-4 Hz is |k| as the frequency domain characteristic parameter to characterize the moisture content of the oil paper. It is evaluating the wetting state of transformer oil paper insulation in a more accurate way by using the dielectric characteristics in frequency domain. The variation of complex permittivity and |k| of oil paper insulation specimens with different moisture content were studied experimentally. The results show that the imaginary part of the complex dielectric constant increases with the increase of the moisture content of the insulating paper gradually in the test frequency and the real part curve moves toward the high frequency section, and the amplitude of the complex dielectric constant of the low frequency section increases with the increase of the moisture content. The |k| of oil paper samples with different moisture content was calculated by fitting the measured ε′-f curve. It is found that there is a better exponential function relationship between |k| and moisture content of oil paper, which can be used for quantitative evaluation of moisture content of oil paper insulation. Finally, an experimental example is given to verify the feasibility of evaluating the moisture level of oil paper insulation by using characteristic parameter |k|.