Transformer oil-paper insulation aging is an important factor affecting the normal operation of transformers. At present

the depolarization current（PDC） method based on the time domain detection is widely used in transformer insulation condition diagnosis

but the feature extraction of depolarizing current line is still in the initial stage. According to the dielectric response and trap theory

we put forward a method to draw the depolarizing trap density spectrum

extract two features including peak value Smax and peak time constant Tmax

and then use them to evaluate the state of transformer insulation. Firstly

different polarization voltage tests are used to analyze the relationship between new characteristic value and polarization voltage. Secondly

the trap density of transformers with different furfural concentration are comparatively analyzed. Finally

a large number of instances on oil-paper insulation aging diagnosis are verified and analyzed. The research results show that the peak value is positively related with furfural content

and peak time constant is negatively related with furfural content. With the deepening of oil-paper insulation aging

the peak value Smax increases gradually

but the peak time constant Tmax decreases gradually. The value range of two characteristics proposed can be used as the basis of the diagnosis of transformer insulation aging status.