A method for designing an expandable concrete grounding grid is disclosed. A substation grounding grid is formed by splicing a plurality of precast concrete formworks, Each precast concrete formwork is poured by embedding a grounding grid, and the edge of the formwork is overlapped by a step-type bus bar. The grounding grid includes a horizontal grounding grid and a grounding electrode which passes through the concrete formwork vertically downward. The construction of the grounding grid is completed after each concrete formwork is placed stably. The utility model has the advantages that the grounding grid structure is optimized, the grounding grid structure is suitable for the safety performance requirements of soft soil, frozen soil and other geological conditions, the construction periodis short, the protective property of the grounding grid is enhanced, and the operation safety of the grounding equipment is ensured.