Abstract: Under multiple stresses of electricity and heat inside transformer, air bubbles are likely to be generated between turns of the oil-paper insulation to induce partial discharge (PD), which threatens the safe operation of transformer. In this paper, the evolution characteristics of bubble PD between oil and paper insulation layers are studied by the pulse current method, and the relationship between PD evolution process and bubble defect degradation is grasped. The results show that, according to the variation of average apparent charge (Q), discharge repetition rate (N) and partial discharge phase spectrum (PRPD) of PD with time, the bubble PD evolution can be divided into three stages, namely, initial stage, transition stage, and stable stage. The PD evolution mechanism was studied by changing the duration of voltage application and bubble position and analyzing the change of gas content inside the bubble after PD lasted for different time. It is found that the reason for the pattern features at initial stage is the accumulation of space charge on the insulating paper surface, and the evolution of the transitional stage is due to the exhaustion of electronegative gas represented by oxygen under the continuous effect of PD, which changes the discharge mode in the bubble change. In addition, the interlayer position, the height and area of the bubbles have significant impacts on the apparent charge and repetition rate. Quantitative results show that the intensity of PD is positively correlated with the bubble defect parameters, and the degree of defect deterioration can be evaluated by PD. This study reveals the evolution law and influencing factors of interlayer bubble PD, which can provide a reference for the evaluation of transformer oil-paper insulation state.