The grounding system of gas insulated metal enclosed transmission line ( shorted for GIL) is very important for the stable and reliable operation of the transmission mode． The grounding system of an actual 500 kV GIL project is studied based on CDEGS． Its grounding system model is established，and the electrical characteristic parameters such as earth resistance，step voltage，touch voltage and earth potential rise are calculated and analyzed． The influence rules of the depth of grounding grid， the concrete resistivity，the number of earth points and the main structure steel on the electrical characteristics of GIL grounding system are studied． The results show that the reinforced concrete structure as a natural grounding body has the greatest influence on the electrical characteristics of GIL，and the number of earth points has the least impact on the electrical characteristics of GIL． The optimized grounding system of GIL provides a basis for the construction and operation of GIL utility tunnel