In order to determine insulation thickness of DC cable in Zhangbei HVDC flexible project, a design method of insulation thickness of the DC cable is presented. The influences of life index, breakdown field intensity conversion coefficient and Bahder coefficient on insulation thickness are analyzed. The insulation thickness of ±535 kV DC cable is calculated by establishing equations. The electric field distribution and electric field distortion rate under different conductivity temperature coefficient electric conductivity electric field coefficient and insulation temperature difference are calculated by finite element simulation. The results show that the insulation electric field distortion rate of ± 535 kV DC cable meets the requirements of national standards under different conductivity temperature coefficient, conductivity electric field coefficient and insulation temperature difference, and the rationality of the design method is verified. The research results provide theoretical guidance for the insulation thickness design and validation of DC cable and provide technical support for the construction of Zhangbei project.