In order to realize the localization of high voltage direct current (HVDC) cable insulating materials, the insulating materials of HVDC flexible cable with the high carry capacity (90℃ of the working temperature) are prepared by nano filler doping method in this paper. The key parameters including space charge, resistivity, DC breakdown strength are investigated, and the results are compared with that of HVAC insulating materials and external HVDC insulating materials. It is found that the developed insulating materials have good space charge suppression performance at 20℃ and high temperature (90℃), and the electric field distortion ratio is less than 5%. Comparing to the two other materials, its resistivity increases by 10 times and the resistivity temperature coefficient decreases by 1/3 at high temperature, so it decreases the insulation heat loss and fits to the design of the cable system. Its DC breakdown strength at high temperature is 1.27~1.6 times as much as that of the two other materials. The research shows that the materials are suitable for the insulation of HVDC flexible cable with the working temperature of 90℃ for its perfect electrical performance.