Electric field optimization plays a significant role in the insulation design of gaseous insulated pipeline (GIL) for the DC power system. Functionally graded material (FGM) with spatial distribution of conductivity σ (σ-FGM) provides a potential method to uniform the electric field and thus reduces flashovers to some extent. However, researches at present are nearly at a standstill for the complicated fabrication techniques and huge bulk conduction losses. In this paper, a novel cone-type insulator with surface conductivity gradient was proposed based on direct fluorination treatment. Theoretical analysis shows that the electric field distortion near the high voltage electrode can be effectively suppressed with increasing the thickness and conductivity of the fluorinated layer. Epoxy cone-type insulators with homogeneous and graded surface conductivity were then prepared by different fluorination treatments to verify the effectiveness of σ-FGM design. As for the homogeneously fluorinated cone-type insulators, the DC flashover characteristics were improved with the rise of fluorination time. Especially, the flashover voltage of the insulators with σ-FGM layers can be raised by up to 36.3 % compared with that of the traditional ones, owing to a more uniform electric field distribution. Theoretical and experimental results prove that the insulator with surface σ-FGM is a promising choice for DC compact GIL.