The air gap at the interface inside the cable terminations for high-speed trains and the partial discharge caused by it are two important factors affecting the insulating performance. The development of air gap and partial discharge will eventually lead to breakdown faults. To investigate the evolutionary characteristics of the air gap and partial discharge

the simulation models and samples of cable terminations containing defects are constructed in this paper. By analysing the variation law of the electric field and the multidimensional information of partial discharge

the evolution process of the air gap is divided into four well-characterised stages. Especially in the third stage

the partial discharge extinction voltage is 51.41% lower than that of the defect-free samples and even lower than the working voltage. The asymmetry of discharge is the most significant factor. The volume of discharge in the third quadrant is significantly higher than that in the first quadrant. This important feature can be applied to the inspection and evaluation of the insulating state of the cable terminations. The partial discharge characteristics of the air gap revealed in this paper are proposed to provide an important theoretical supplement to the study of interface discharges between heterogeneous dielectrics.