Cavity induced discharge has been acknowledged one of the most common form of partial discharge（PD）in transformers. Figuring out the PD evolutional behavior of oil-paper insulation cavity defect under various agingconditions would contribute to the quantitative evaluation of risk in transformers. To investigate the developing char-acteristics of the cavity induced discharge and offer the parameters charactering aging degree and discharge risk ofthe insulation

four oil-paper insulation samples with different aging degrees were prepared by accelerating thermallyaging procedures

and a PD detection platform of oil-paper insulation was established to obtain the PRPD pattern.Conclusions can be drawn that: PD inception voltage and breakdown voltage show a decreasing trend with aging con-dition;the Weibull scale parameter α and average discharge power P change remarkably;and the time interval（Δt）between two consecutive discharges shows a tight correlation with the discharging stage and aging condition of the insulation

indicating that it is applicable to quantitative risk assessment of the insulation in the presence of PD.