Abstract: The surface coatings of nonlinear conductive materials have excellent surface charge suppression ability and can adaptively regulate the resistive electric field, which has attracted a lot of attention in the field of DC gas insulated switchgear (GIS) gas-solid insulator structure design; however, the coatings prepared by dipping, brushing or spraying have the problems of low surface finish, poor uniformity and poor reproducibility, which will directly affect the suppression effect of the coating on the surface charge. In this paper, a method based on "two-step casting" is proposed for the preparation of insulator surface coatings, based on which nonlinear conductive materials are coated on the surface of conical insulators. The proposed method is compared with the dipping, brushing and spraying methods by means of surface morphology tests, surface charge accumulation and DC flashover experiments. The results show that coating surface obtained by the "two-step casting" method is smooth and uniform without obvious defects, the surface roughness is significantly decreased, and the optimum surface charge suppression effect can be achieved compared with other methods. Under −20 kV DC voltage, the coating obtained by the "two-step casting" can reduce the total amount of positive and negative charge accumulation by 62.7%, 68.6%, 77.9%, and 66.7%, 20%, 67.7% compared with the dipping, brushing, and spraying methods, respectively. In terms of DC along-surface flashover characteristics, the positive and negative polarization characteristics along-surface flashover voltages of the coatings obtained by the two-step casting method are improved by 2.7%, 6.3%, 20.7%, and 4.2%, 4.7%, 9.9% compared with those methods. Further, this paper analyzes the above experimental phenomena by combining the test results of surface roughness and surface trap distribution, and concludes that the difference in surface roughness of the specimens under different coating methods is the main reason for the difference in surface charge distribution, reducing the surface roughness will make the surface trap energy level and density of the specimen smaller as well, thus accelerating the process of charge dissipation on the coating surface.