The insulation material and the structure design of power cables in the high voltage direct current（HVDC）Flexible transmission system are discussed at length.It is pointed out that space charge is a hard problem with HVDC Flexible cable insulation.Both the development of space charge measuring devices for cable insulation，and the measuring of the energy distribution for space charge contribute to the investigation of space charge conduct.During the process of nano powder doping for suppression of space charge，the surface physical modification and chemical modification contribute to the compatibility between nano particles and cross linked polyethylene（XLPE）molecules.It is indicated that the electric field is proportional to the volume resistivity for HVDC Flexible cable insulation，while the volume resistivity depends on the temperature and field intensity.The inverse impulse voltage borne by the HVDC Flexible cable during running is the critical factor because of the effect of space charge.Finally，the development of90℃insulation material for the HVDC Flexible cable is developed，and the environment protective structures of land and submarine cables are designed for the HVDC Flexible transmission system.