Based on the allowable safety current for human body, the maximum allowable values of the safety indexs under the conditions of power frequency short circuit and lightning strike are caculated. Taking two typical designs of grounding device for existing 220 kV urban transmission steel tubular tower as prototypes, the actual values of safety index of grounding device under typical design are obtained by the simulation on CDEGS software. Combined with the actual situation, it is considered that the power frequency step voltage exceeding the standard is the key problem for the safety of grounding devices. A method of replacing part of the vertical grounding electrode with spiral grounding module is proposed to balance the step voltage, and the structure of spiral grounding module is optimized according to the change law of its grounding characteristics with its structural parameters. A design method of combining two typical designs of grounding device for 220 kV urban transmission steel tubular tower with the spiral grounding module is proposed to meet the safety design index, and the optimization scheme of grounding safety design for transmission steel tubular tower is obtained. The research results provide a method reference for the safety design of transmission tower grounding.