The modification of insulating materials is regarded as an important way to improve the insulation performance of DC spacers. Many simulations have been carried out to guide the development of insulating materials. However, the impacts of temperature on surface charge accumulation and electric field distributions are neglected in many simulations. Since the conductivities are strongly impacted by temperature, the thermal stresses should be considered in the simulations of charge accumulation. To solve these problems, a coupled electro-thermal field simulation model of charge accumulation under DC stress is proposed in this paper. Then, a DC spacer model is introduced as an example, and the parameters in the simulation are shown. Based on the simulation model, the effects of the spacer volume conductivity on the distribution of the surface charge, the saturation time of charge accumulation and the distribution of the electric field along the spacer are investigated. In addition, the influencing mechanism is explained. The results indicate that the volume conductivity of the insulating material should be decreased by two orders of magnitude from the existing AC materials to reduce the surface charge accumulation. With the reduced volume conductivity, 1) the peak value of the charge density on the upper surface decreases 52.9%, and that of lower surface decreases 51.0%; 2) the peak value of the tangential component of the electric field strength along the cone-type spacer in DC-GIL decreases by 12.2%; and 3) the duration of the capacitive-resistive field transition increases by 38.5-fold. The simulation method and advice on the material volume conductivity can be referenced in the DC space design.