Abstract: In order to investigate the partial discharge characteristics and degradation mechanism of EPR cable insulation after thermal-oxidative aging, the partial discharge tests of EPR samples with different aging degrees were carried out by typical column-plate electrodes, and the discharge development characteristics and physical and chemical properties of the materials were analyzed. The results show that the initial discharge voltage and flashover voltage decrease with the increase of insulation aging degree, and the discharge process can be divided into four stages according to the variation law of discharge characteristics during the discharge process: initial stage, development stage, continuous stage, and pre-flashover stage, and the PRPD spectra of samples with different aging degree show specific shapes in each stage. Surface gasification or gas desorption and the precipitation of additives lead to the appearance of "gully zone", "white block" and "micropore" on the surface of the sample. The increase of the number of free radicals such as carbonyl groups, polyethylene and polypropylene in the molecular structure is the key reason why EPR insulation is more prone to partial discharge after thermal oxygen aging.