The safety margin of high voltage direct current (HVDC) cable system is the precondition to ensure the long-term safe operation of cable lines. To obtain the highest service voltage of the HVDC cable system,the conductivity characteristics of insulation of cables and accessories are tested in this paper,and the dependence of conductivity on temperature and electric field intensity and the expression of conductivity are obtained. A safety margin test method is proposed based on ±80 kV HVDC cable system,the breakdown voltage of the HVDC cable system at the maximum operating temperature of 90℃ is tested by applying voltage step by step. The electric field distribution of cable system breakdown is calculated according to the conductivity of cable insulation and accessory insulation,and the safety margin of the cable system is obtained by comparing the electric field intensity during the breakdown of the cable system with the electric field intensity required for long-term operation. The research shows that the safety margin test method proposed in this paper can obtain the safety margin of HVDC cable system,and the research results provide theoretical and experimental basis for the safe operation of HVDC cable engineering.