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A recurring challenge of a DC SF<inf>6</inf>/N<inf>2</inf>-filled GIS/GIL apparatus is the charge accumulation at DC stress. The conventional design rules and knowledge of AC spacers may not be applicable for this new type of apparatus. A novel design rule is proposed considering the effect of accumulated charge on the threshold of electric field strength being resistant to the superposed voltage. A surface charge accumulation simulation model is introduced, and the key parameters in the simulation model are measured. In addition, an experimental platform for a 100 kV spacer flashover test is established. Spacer flashover tests under superimposed voltage with opposing polarities are carried out, and the withstanding voltage of the spacer is obtained. Finally, based on the proposed model, the threshold of the surface electric field strength (tangential component) on the DC spacer in SF<inf>6</inf>/N<inf>2</inf> mixed gases is discussed. For a reliable insulation design of a DC GIS/GIL apparatus filled with 0.7 MPa SF<inf>6</inf>/N<inf>2</inf>, the threshold of surface electric field strength on the DC spacer is 12 kV/mm. The insulation design rule can be referenced in the design of a high-voltage DC SF<inf>6</inf>/N<inf>2</inf>-filled GIS/GIL apparatus.