The main aim of this paper is to investigate the evolutionary characteristics of surface discharge within the oil-paper insulation under the different aging conditions and extract parameters which are capable of characterizing the risk of discharge events.Oil-paper specimens of four kinds of different aging intervals are obtained through thermally accelerating aging processes and the developing processes of partial discharge（PD）

and pulse sequences are acquired under step-up applied AC voltage.The results show that the current pulses of the surface discharge usually appear in sequence with growing amplitude and the rise/fall time of pluses are faster than the counterparts in gas.The aging conditions of specimens hardly affect the partial discharge inception voltage（PDIV） and breakdown voltage（BDV）

however

the parameters P and α are sensitive to the oil/paper conditions.The interval between consecutive discharges

Δt

is strongly dependent on discharge stage and aging condition of the insulation

and it can be used as the key parameter to the risk assessment of the PD.