With the development of high voltage directly current (DC) transmission technique, DC gas insulated switchgear (GIS) is attracting more and more attentions. DC GIS has the advantages of compact size, high reliability and interchangeability. Compared with alternating current (AC), the electric field distribution of the DC GIS insulation system depends on the conductivity of the dielectric. In the insulation system, the electric field from the capacitive into resistance at the same time, the charge at the interface of gas-solid interface accumulation and change the original electric field, resulting in its flashover characteristics along the surface decreased and hence restricts the application of dc GIS in engineering. This contribution presents a numerical model that is appropriate for the calculation of field distributions and surface charge accumulation on solid insulation in a DC GIS. The model studies the charge transport process in the gas and solid insulation. Based on the model, the influence of gas ion pair generation rate, bulk and surface conductivity of solid insulation on the polarity and distribution of surface charge. This study will help improve the insulation design of the DC GIS by reducing the critical area electric field and minimizing the surface charge accumulation to optimize the geometry and material properties of the insulator and verify the changes of design.