The utility model discloses a waterproof grounding main line of an underground substation. The waterproof grounding main line comprises a conductor used for introducing the current to a grounding network cable, a seamless steel pipe, insulation filling materials, nonmetal stopping plugs and water-stopping discs, the insulation filling materials are filled in the seamless steel pipe, the two nonmetal stopping plugs are correspondingly arranged at two end parts of the seamless steel pipe, the water-stopping discs are arranged at the peripheral side of the seamless steel pipe in a winding manner, the nonmetal stopping plugs are axially provided with through holes allowing the conductor to penetrate through, the conductor extends in the axial direction of the seamless steel pipe and is arranged in the seamless steel pipe in a penetrating manner, and two end parts of the conductor respectively penetrate out of the through holes of the nonmetal stopping plugs. The waterproof grounding main line is applicable to the grounding of the underground substation, the high grounding current can pass through the waterproof grounding main line, the waterproof requirement is met, the problem that a grounding leading wire passes through a large base plate of the underground substation is well solved, and the waterproof grounding main line can be customized in batches as a product and can be employed as the grounding leading wire when the large base plate of the underground substation is cast.