In high voltage direct current cable system,the cable joint and termination are the weakness to stand high voltage,thus making their breakdown strength the critical property to the reliability of power transmission system. In this work,the breakdown mechanism of cross linked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) interface in the cable accessory during the load cycle test of ±320 kV HVDC cable is analyzed. Firstly,the voltage withstand test of the cable accessory suggests that the weakness locates at the XLPE/EPDM interfaces under applied electric field. Secondly,the temperature and electric field distribution under no load and full load are calculated through finite element simulation. This result indicates that the highest electric field is about 29.5 kV/mm located at the cable insulation material near the stress cone,which is far below the breakdown strength of each insulation material. Finally,the electric field distortion in the interface of XLPE and EPDM induced by space charge is measured by a pulsed electro-acoustic system. It is found that the electric field distortion rate reaches 100%~200% in the interface,suggesting that the space charge in the interface is responsible for the breakdown behavior of accessories. This work might provide a breakdown mechanism for further development of high voltage materials and structures for HVDC cables accessories.