In the test of the maximum 900 kV DC superimposed 1 800 kV operating or 2 400 kV lightning impulse voltage, a special resistor element needs to be designed to protect the DC voltage generator from being damaged by the impulse voltage, and the appropriate shape is designed of the resistor to prevent flashover. Based on the simulation calculations, the resistance value is selected in the DC superimposed impact test circuit. And based on the external insulation flashover occurred during the test, the resistance wire winding method and the electrode shape are optimized. The results show that the optimum protection resistor is 500 kΩ which can meet the role of protecting the DC voltage generator in the test circuit; The externally insulated length is selected to be 10 m, the resistance wire winding method uses two resistance wires for double-wire winding, The high-voltage terminal electrode is equipped with ring diameter 2 700 mm, hood depth 500 mm and pipe diameter 250 mm of a grading ring on the high-voltage end, which can significantly improve voltage and electric field distribution on the resistor, and meet the requirement of no external insulation flashover and prevent flashover along the surface.